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    Linux fun book Linux fun book Document Transcript

    • Linux Fundamentals Paul Cobbaut
    • Linux Fundamentals Paul Cobbaut lt-2.0 Published Thu 01 Aug 2013 01:00:39 CEST Abstract This book is meant to be used in an instructor-led training. For self-study, the intent is to read this book next to a working Linux computer so you can immediately do every subject, practicing each command. This book is aimed at novice Linux system administrators (and might be interesting and useful for home users that want to know a bit more about their Linux system). However, this book is not meant as an introduction to Linux desktop applications like text editors, browsers, mail clients, multimedia or office applications. More information and free .pdf available at http://linux-training.be . Feel free to contact the author: • Paul Cobbaut: paul.cobbaut@gmail.com, http://www.linkedin.com/in/cobbaut Contributors to the Linux Training project are: • Serge van Ginderachter: serge@ginsys.eu, build scripts and infrastructure setup • Ywein Van den Brande: ywein@crealaw.eu, license and legal sections • Hendrik De Vloed: hendrik.devloed@ugent.be, buildheader.pl script We'd also like to thank our reviewers: • Wouter Verhelst: wo@uter.be, http://grep.be • Geert Goossens: mail.goossens.geert@gmail.com, http://www.linkedin.com/in/geertgoossens • Elie De Brauwer: elie@de-brauwer.be, http://www.de-brauwer.be • Christophe Vandeplas: christophe@vandeplas.com, http://christophe.vandeplas.com • Bert Desmet: bert@devnox.be, http://blog.bdesmet.be • Rich Yonts: richyonts@gmail.com, Copyright 2007-2013 Netsec BVBA, Paul Cobbaut Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled 'GNU Free Documentation License'.
    • Table of Contents I. introduction to Linux ........................................................................................ 1 1. Linux history ............................................................................................... 2 2. distributions ................................................................................................. 4 3. licensing ...................................................................................................... 6 4. getting Linux at home ............................................................................... 10 II. first steps on the command line .................................................................... 21 5. man pages ................................................................................................. 22 6. working with directories ........................................................................... 26 7. working with files ..................................................................................... 35 8. working with file contents ........................................................................ 44 9. the Linux file tree ..................................................................................... 51 III. shell expansion .............................................................................................. 72 10. commands and arguments ....................................................................... 73 11. control operators ..................................................................................... 83 12. variables .................................................................................................. 89 13. shell history ........................................................................................... 100 14. file globbing .......................................................................................... 106 IV. pipes and commands .................................................................................. 113 15. redirection and pipes ............................................................................. 114 16. filters ..................................................................................................... 123 17. basic Unix tools .................................................................................... 136 V. vi ..................................................................................................................... 145 18. Introduction to vi .................................................................................. 146 VI. scripting ....................................................................................................... 156 19. scripting introduction ............................................................................ 157 20. scripting loops ....................................................................................... 163 21. scripting parameters .............................................................................. 170 22. more scripting ....................................................................................... 178 VII. local user management ............................................................................. 186 23. users ...................................................................................................... 187 24. groups .................................................................................................... 207 VIII. file security ............................................................................................... 213 25. standard file permissions ...................................................................... 214 26. advanced file permissions ..................................................................... 225 27. access control lists ................................................................................ 231 28. file links ................................................................................................ 235 IX. Appendices ................................................................................................... 242 A. certifications ........................................................................................... 243 B. keyboard settings .................................................................................... 245 C. hardware ................................................................................................. 247 D. License ................................................................................................... 251 Index .................................................................................................................... 258 iii
    • List of Tables 18.1. getting to command mode ......................................................................... 18.2. switch to insert mode ................................................................................. 18.3. replace and delete ...................................................................................... 18.4. undo and repeat .......................................................................................... 18.5. cut, copy and paste a line .......................................................................... 18.6. cut, copy and paste lines ............................................................................ 18.7. start and end of line ................................................................................... 18.8. join two lines ............................................................................................. 18.9. words .......................................................................................................... 18.10. save and exit vi ........................................................................................ 18.11. searching .................................................................................................. 18.12. replace ...................................................................................................... 18.13. read files and input .................................................................................. 18.14. text buffers ............................................................................................... 18.15. multiple files ............................................................................................ 18.16. abbreviations ............................................................................................ 23.1. Debian User Environment .......................................................................... 23.2. Red Hat User Environment ........................................................................ 25.1. Unix special files ....................................................................................... 25.2. standard Unix file permissions .................................................................. 25.3. Unix file permissions position ................................................................... 25.4. Octal permissions ....................................................................................... iv 147 147 148 148 148 149 149 149 150 150 151 151 151 152 152 152 206 206 216 217 217 220
    • Part I. introduction to Linux
    • Chapter 1. Linux history Table of Contents 1.1. Linux history .................................................................................................... 3 This chapter briefly tells the history of Unix and where Linux fits in. If you are eager to start working with Linux without this blah, blah, blah over history, distributions, and licensing then jump straight to Part II - Chapter 6. Working with Directories page 26. 2
    • Linux history 1.1. Linux history All modern operating systems have their roots in 1969 when Dennis Ritchie and Ken Thompson developed the C language and the Unix operating system at AT&T Bell Labs. They shared their source code (yes, there was open source back in the Seventies) with the rest of the world, including the hippies in Berkeley California. By 1975, when AT&T started selling Unix commercially, about half of the source code was written by others. The hippies were not happy that a commercial company sold software that they had written; the resulting (legal) battle ended in there being two versions of Unix in the Seventies : the official AT&T Unix, and the free BSD Unix. In the Eighties many companies started developing their own Unix: IBM created AIX, Sun SunOS (later Solaris), HP HP-UX and about a dozen other companies did the same. The result was a mess of Unix dialects and a dozen different ways to do the same thing. And here is the first real root of Linux, when Richard Stallman aimed to end this era of Unix separation and everybody re-inventing the wheel by starting the GNU project (GNU is Not Unix). His goal was to make an operating system that was freely available to everyone, and where everyone could work together (like in the Seventies). Many of the command line tools that you use today on Linux or Solaris are GNU tools. The Nineties started with Linus Torvalds, a Swedish speaking Finnish student, buying a 386 computer and writing a brand new POSIX compliant kernel. He put the source code online, thinking it would never support anything but 386 hardware. Many people embraced the combination of this kernel with the GNU tools, and the rest, as they say, is history. Today more than 90 percent of supercomputers (including the complete top 10), more than half of all smartphones, many millions of desktop computers, around 70 percent of all web servers, a large chunk of tablet computers, and several appliances (dvdplayers, washing machines, dsl modems, routers, ...) run Linux. It is by far the most commonly used operating system in the world. Linux kernel version 3.2 was released in January 2012. Its source code grew by almost two hundred thousand lines (compared to version 3.1) thanks to contributions of over 4000 developers paid by about 200 commercial companies including Red Hat, Intel, Broadcom, Texas Instruments, IBM, Novell, Qualcomm, Samsung, Nokia, Oracle, Google and even Microsoft. http://en.wikipedia.org/wiki/Dennis_Ritchie http://en.wikipedia.org/wiki/Richard_Stallman http://en.wikipedia.org/wiki/Linus_Torvalds http://kernel.org http://lwn.net/Articles/472852/ http://www.linuxfoundation.org/ http://en.wikipedia.org/wiki/Linux http://www.levenez.com/unix/ (a huge Unix history poster) 3
    • Chapter 2. distributions Table of Contents 2.1. 2.2. 2.3. 2.4. 2.5. Red Hat ............................................................................................................ Ubuntu .............................................................................................................. Debian .............................................................................................................. Other ................................................................................................................ Which to choose ? ........................................................................................... 5 5 5 5 5 This chapter gives a short overview of current Linux distributions. A Linux distribution is a collection of (usually open source) software on top of a Linux kernel. A distribution (or short, distro) can bundle server software, system management tools, documentation and many desktop applications in a central secure software repository. A distro aims to provide a common look and feel, secure and easy software management and often a specific operational purpose. Let's take a look at some popular distributions. 4
    • distributions 2.1. Red Hat Red Hat is a billion dollar commercial Linux company that puts a lot of effort in developing Linux. They have hundreds of Linux specialists and are known for their excellent support. They give their products (Red Hat Enterprise Linux and Fedora) away for free. While Red Hat Enterprise Linux (RHEL) is well tested before release and supported for up to seven years after release, Fedora is a distro with faster updates but without support. 2.2. Ubuntu Canonical started sending out free compact discs with Ubuntu Linux in 2004 and quickly became popular for home users (many switching from Microsoft Windows). Canonical wants Ubuntu to be an easy to use graphical Linux desktop without need to ever see a command line. Of course they also want to make a profit by selling support for Ubuntu. 2.3. Debian There is no company behind Debian. Instead there are thousands of well organised developers that elect a Debian Project Leader every two years. Debian is seen as one of the most stable Linux distributions. It is also the basis of every release of Ubuntu. Debian comes in three versions: stable, testing and unstable. Every Debian release is named after a character in the movie Toy Story. 2.4. Other Distributions like CentOS, Oracle Enterprise Linux and Scientific Linux are based on Red Hat Enterprise Linux and share many of the same principles, directories and system administration techniques. Linux Mint, Edubuntu and many other *buntu named distributions are based on Ubuntu and thus share a lot with Debian. There are hundreds of other Linux distributions. 2.5. Which to choose ? When you are new to Linux in 2012, go for the latest Ubuntu or Fedora. If you only want to practice the Linux command line then install one Ubuntu server and/or one CentOS server (without graphical interface). redhat.com ubuntu.com debian.org centos.org distrowatch.com 5
    • Chapter 3. licensing Table of Contents 3.1. 3.2. 3.3. 3.4. 3.5. 3.6. 3.7. 3.8. about software licenses .................................................................................... public domain software and freeware .............................................................. Free Software or Open Source Software ......................................................... GNU General Public License .......................................................................... using GPLv3 software ..................................................................................... BSD license ..................................................................................................... other licenses ................................................................................................... combination of software licenses ..................................................................... 7 7 8 8 8 9 9 9 This chapter briefly explains the different licenses used for distributing operating systems software. Many thanks go to Ywein Van den Brande for writing most of this chapter. Ywein is an attorney at law, co-author of The International FOSS Law Book and author of Praktijkboek Informaticarecht (in Dutch). http://ifosslawbook.org http://www.crealaw.eu 6
    • licensing 3.1. about software licenses There are two predominant software paradigms: Free and Open Source Software (FOSS) and proprietary software. The criteria for differentiation between these two approaches is based on control over the software. With proprietary software, control tends to lie more with the vendor, while with Free and Open Source Software it tends to be more weighted towards the end user. But even though the paradigms differ, they use the same copyright laws to reach and enforce their goals. From a legal perspective, Free and Open Source Software can be considered as software to which users generally receive more rights via their license agreement than they would have with a proprietary software license, yet the underlying license mechanisms are the same. Legal theory states that the author of FOSS, contrary to the author of public domain software, has in no way whatsoever given up his rights on his work. FOSS supports on the rights of the author (the copyright) to impose FOSS license conditions. The FOSS license conditions need to be respected by the user in the same way as proprietary license conditions. Always check your license carefully before you use third party software. Examples of proprietary software are AIX from IBM, HP-UX from HP and Oracle Database 11g. You are not authorised to install or use this software without paying a licensing fee. You are not authorised to distribute copies and you are not authorised to modify the closed source code. 3.2. public domain software and freeware Software that is original in the sense that it is an intellectual creation of the author benefits copyright protection. Non-original software does not come into consideration for copyright protection and can, in principle, be used freely. Public domain software is considered as software to which the author has given up all rights and on which nobody is able to enforce any rights. This software can be used, reproduced or executed freely, without permission or the payment of a fee. Public domain software can in certain cases even be presented by third parties as own work, and by modifying the original work, third parties can take certain versions of the public domain software out of the public domain again. Freeware is not public domain software or FOSS. It is proprietary software that you can use without paying a license cost. However, the often strict license terms need to be respected. Examples of freeware are Adobe Reader, Skype and Command and Conquer: Tiberian Sun (this game was sold as proprietary in 1999 and is since 2011 available as freeware). 7
    • licensing 3.3. Free Software or Open Source Software Both the Free Software (translates to vrije software in Dutch and to Logiciel Libre in French) and the Open Source Software movement largely pursue similar goals and endorse similar software licenses. But historically, there has been some perception of differentiation due to different emphases. Where the Free Software movement focuses on the rights (the four freedoms) which Free Software provides to its users, the Open Source Software movement points to its Open Source Definition and the advantages of peer-to-peer software development. Recently, the term free and open source software or FOSS has arisen as a neutral alternative. A lesser-used variant is free/libre/open source software (FLOSS), which uses libre to clarify the meaning of free as in freedom rather than as in at no charge. Examples of free software are gcc, MySQL and gimp. Detailed information about the four freedoms can be found here: http://www.gnu.org/philosophy/free-sw.html The open source definition can be found at: http://www.opensource.org/docs/osd The above definition is based on the Debian Free Software Guidelines available here: http://www.debian.org/social_contract#guidelines 3.4. GNU General Public License More and more software is being released under the GNU GPL (in 2006 Java was released under the GPL). This license (v2 and v3) is the main license endorsed by the Free Software Foundation. It’s main characteristic is the copyleft principle. This means that everyone in the chain of consecutive users, in return for the right of use that is assigned, needs to distribute the improvements he makes to the software and his derivative works under the same conditions to other users, if he chooses to distribute such improvements or derivative works. In other words, software which incorporates GNU GPL software, needs to be distributed in turn as GNU GPL software (or compatible, see below). It is not possible to incorporate copyright protected parts of GNU GPL software in a proprietary licensed work. The GPL has been upheld in court. 3.5. using GPLv3 software You can use GPLv3 software almost without any conditions. If you solely run the software you even don’t have to accept the terms of the GPLv3. However, any other use - such as modifying or distributing the software - implies acceptance. 8
    • licensing In case you use the software internally (including over a network), you may modify the software without being obliged to distribute your modification. You may hire third parties to work on the software exclusively for you and under your direction and control. But if you modify the software and use it otherwise than merely internally, this will be considered as distribution. You must distribute your modifications under GPLv3 (the copyleft principle). Several more obligations apply if you distribute GPLv3 software. Check the GPLv3 license carefully. You create output with GPLv3 software: The GPLv3 does not automatically apply to the output. 3.6. BSD license There are several versions of the original Berkeley Distribution License. The most common one is the 3-clause license ("New BSD License" or "Modified BSD License"). This is a permissive free software license. The license places minimal restrictions on how the software can be redistributed. This is in contrast to copyleft licenses such as the GPLv. 3 discussed above, which have a copyleft mechanism. This difference is of less importance when you merely use the software, but kicks in when you start redistributing verbatim copies of the software or your own modified versions. 3.7. other licenses FOSS or not, there are many kind of licenses on software. You should read and understand them before using any software. 3.8. combination of software licenses When you use several sources or wishes to redistribute your software under a different license, you need to verify whether all licenses are compatible. Some FOSS licenses (such as BSD) are compatible with proprietary licenses, but most are not. If you detect a license incompatibility, you must contact the author to negotiate different license conditions or refrain from using the incompatible software. 9
    • Chapter 4. getting Linux at home Table of Contents 4.1. 4.2. 4.3. 4.4. 4.5. download a Linux CD image ......................................................................... download Virtualbox ..................................................................................... create a virtual machine ................................................................................. attach the CD image ...................................................................................... install Linux ................................................................................................... 11 11 12 17 20 This book assumes you have access to a working Linux computer. Most companies have one or more Linux servers, if you have already logged on to it, then you 're all set (skip this chapter and go to the next). Another option is to insert a Ubuntu Linux CD in a computer with (or without) Microsoft Windows and follow the installation. Ubuntu will resize (or create) partitions and setup a menu at boot time to choose Windows or Linux. If you do not have access to a Linux computer at the moment, and if you are unable or unsure about installing Linux on your computer, then this chapter proposes a third option: installing Linux in a virtual machine. Installation in a virtual machine (provided by Virtualbox) is easy and safe. Even when you make mistakes and crash everything on the virtual Linux machine, then nothing on the real computer is touched. This chapter gives easy steps and screenshots to get a working Ubuntu server in a Virtualbox virtual machine. The steps are very similar to installing Fedora or CentOS or even Debian, and if you like you can also use VMWare instead of Virtualbox. 10
    • getting Linux at home 4.1. download a Linux CD image Start by downloading a Linux CD image (an .ISO file) from the distribution of your choice from the Internet. Take care selecting the correct cpu architecture of your computer; choose i386 if unsure. Choosing the wrong cpu type (like x86_64 when you have an old Pentium) will almost immediately fail to boot the CD. 4.2. download Virtualbox Step two (when the .ISO file has finished downloading) is to download Virtualbox. If you are currently running Microsoft Windows, then download and install Virtualbox for Windows! 11
    • getting Linux at home 4.3. create a virtual machine Now start Virtualbox. Contrary to the screenshot below, your left pane should be empty. Click New to create a new virtual machine. We will walk together through the wizard. The screenshots below are taken on Mac OSX; they will be slightly different if you are running Microsoft Windows. 12
    • getting Linux at home Name your virtual machine (and maybe select 32-bit or 64-bit). Give the virtual machine some memory (512MB if you have 2GB or more, otherwise select 256MB). 13
    • getting Linux at home Select to create a new disk (remember, this will be a virtual disk). If you get the question below, choose vdi. 14
    • getting Linux at home Choose dynamically allocated (fixed size is only useful in production or on really old, slow hardware). Choose between 10GB and 16GB as the disk size. 15
    • getting Linux at home Click create to create the virtual disk. Click create to create the virtual machine. 16
    • getting Linux at home 4.4. attach the CD image Before we start the virtual computer, let us take a look at some settings (click Settings). Do not worry if your screen looks different, just find the button named storage. 17
    • getting Linux at home Remember the .ISO file you downloaded? Connect this .ISO file to this virtual machine by clicking on the CD icon next to Empty. Now click on the other CD icon and attach your ISO file to this virtual CD drive. 18
    • getting Linux at home Verify that your download is accepted. If Virtualbox complains at this point, then you probably did not finish the download of the CD (try downloading it again). It could be useful to set the network adapter to bridge instead of NAT. Bridged usually will connect your virtual computer to the Internet. 19
    • getting Linux at home 4.5. install Linux The virtual machine is now ready to start. When given a choice at boot, select install and follow the instructions on the screen. When the installation is finished, you can log on to the machine and start practising Linux! 20
    • Part II. first steps on the command line
    • Chapter 5. man pages Table of Contents 5.1. man $command .............................................................................................. 5.2. man $configfile .............................................................................................. 5.3. man $daemon ................................................................................................. 5.4. man -k (apropos) ............................................................................................ 5.5. whatis ............................................................................................................. 5.6. whereis ........................................................................................................... 5.7. man sections ................................................................................................... 5.8. man $section $file .......................................................................................... 5.9. man man ........................................................................................................ 5.10. mandb ........................................................................................................... 23 23 23 23 23 24 24 24 24 25 This chapter will explain the use of man pages (also called manual pages) on your Unix or Linux computer. You will learn the man command together with related commands like whereis, whatis and mandb. Most Unix files and commands have pretty good man pages to explain their use. Man pages also come in handy when you are using multiple flavours of Unix or several Linux distributions since options and parameters sometimes vary. 22
    • man pages 5.1. man $command Type man followed by a command (for which you want help) and start reading. Press q to quit the manpage. Some man pages contain examples (near the end). paul@laika:~$ man whois Reformatting whois(1), please wait... 5.2. man $configfile Most configuration files have their own manual. paul@laika:~$ man syslog.conf Reformatting syslog.conf(5), please wait... 5.3. man $daemon This is also true for most daemons (background programs) on your system.. paul@laika:~$ man syslogd Reformatting syslogd(8), please wait... 5.4. man -k (apropos) man -k (or apropos) shows a list of man pages containing a string. paul@laika:~$ man -k syslog lm-syslog-setup (8) - configure laptop mode to switch syslog.conf ... logger (1) - a shell command interface to the syslog(3) ... syslog-facility (8) - Setup and remove LOCALx facility for sysklogd syslog.conf (5) - syslogd(8) configuration file syslogd (8) - Linux system logging utilities. syslogd-listfiles (8) - list system logfiles 5.5. whatis To see just the description of a manual page, use whatis followed by a string. paul@u810:~$ whatis route route (8) - show / manipulate the IP routing table 23
    • man pages 5.6. whereis The location of a manpage can be revealed with whereis. paul@laika:~$ whereis -m whois whois: /usr/share/man/man1/whois.1.gz This file is directly readable by man. paul@laika:~$ man /usr/share/man/man1/whois.1.gz 5.7. man sections By now you will have noticed the numbers between the round brackets. man man will explain to you that these are section numbers. Executable programs and shell commands reside in section one. 1 2 3 4 5 6 7 8 9 Executable programs or shell commands System calls (functions provided by the kernel) Library calls (functions within program libraries) Special files (usually found in /dev) File formats and conventions eg /etc/passwd Games Miscellaneous (including macro packages and conventions), e.g. man(7) System administration commands (usually only for root) Kernel routines [Non standard] 5.8. man $section $file Therefor, when referring to the man page of the passwd command, you will see it written as passwd(1); when referring to the passwd file, you will see it written as passwd(5). The screenshot explains how to open the man page in the correct section. [paul@RHEL52 ~]$ man passwd [paul@RHEL52 ~]$ man 5 passwd # opens the first manual found # opens a page from section 5 5.9. man man If you want to know more about man, then Read The Fantastic Manual (RTFM). Unfortunately, manual pages do not have the answer to everything... paul@laika:~$ man woman No manual entry for woman 24
    • man pages 5.10. mandb Should you be convinced that a man page exists, but you can't access it, then try running mandb. root@laika:~# mandb 0 man subdirectories contained newer manual pages. 0 manual pages were added. 0 stray cats were added. 0 old database entries were purged. 25
    • Chapter 6. working with directories Table of Contents 6.1. 6.2. 6.3. 6.4. 6.5. 6.6. 6.7. 6.8. 6.9. pwd ................................................................................................................. cd .................................................................................................................... absolute and relative paths ............................................................................. path completion .............................................................................................. ls ..................................................................................................................... mkdir .............................................................................................................. rmdir ............................................................................................................... practice: working with directories ................................................................. solution: working with directories ................................................................. 27 27 28 29 29 31 31 32 33 To explore the Linux file tree, you will need some basic tools. This chapter is small overview of the most common commands to work with directories : pwd, cd, ls, mkdir, rmdir. These commands are available on any Linux (or Unix) system. This chapter also discusses absolute and relative paths and path completion in the bash shell. 26
    • working with directories 6.1. pwd The you are here sign can be displayed with the pwd command (Print Working Directory). Go ahead, try it: Open a command line interface (like gnome-terminal, konsole, xterm, or a tty) and type pwd. The tool displays your current directory. paul@laika:~$ pwd /home/paul 6.2. cd You can change your current directory with the cd command (Change Directory). paul@laika$ paul@laika$ /etc paul@laika$ paul@laika$ /bin paul@laika$ paul@laika$ /home/paul cd /etc pwd cd /bin pwd cd /home/paul/ pwd cd ~ You can pull off a trick with cd. Just typing cd without a target directory, will put you in your home directory. Typing cd ~ has the same effect. paul@laika$ paul@laika$ /etc paul@laika$ paul@laika$ /home/paul paul@laika$ paul@laika$ /home/paul cd /etc pwd cd pwd cd ~ pwd cd .. To go to the parent directory (the one just above your current directory in the directory tree), type cd .. . paul@laika$ pwd /usr/share/games paul@laika$ cd .. paul@laika$ pwd /usr/share To stay in the current directory, type cd . ;-) We will see useful use of the . character representing the current directory later. 27
    • working with directories cd Another useful shortcut with cd is to just type cd - to go to the previous directory. paul@laika$ /home/paul paul@laika$ paul@laika$ /etc paul@laika$ /home/paul paul@laika$ /etc pwd cd /etc pwd cd cd - 6.3. absolute and relative paths You should be aware of absolute and relative paths in the file tree. When you type a path starting with a slash (/), then the root of the file tree is assumed. If you don't start your path with a slash, then the current directory is the assumed starting point. The screenshot below first shows the current directory /home/paul. From within this directory, you have to type cd /home instead of cd home to go to the /home directory. paul@laika$ pwd /home/paul paul@laika$ cd home bash: cd: home: No such file or directory paul@laika$ cd /home paul@laika$ pwd /home When inside /home, you have to type cd paul instead of cd /paul to enter the subdirectory paul of the current directory /home. paul@laika$ pwd /home paul@laika$ cd /paul bash: cd: /paul: No such file or directory paul@laika$ cd paul paul@laika$ pwd /home/paul In case your current directory is the root directory /, then both cd /home and cd home will get you in the /home directory. paul@laika$ / paul@laika$ paul@laika$ /home paul@laika$ paul@laika$ paul@laika$ /home pwd cd home pwd cd / cd /home pwd This was the last screenshot with pwd statements. From now on, the current directory will often be displayed in the prompt. Later in this book we will explain how the shell variable $PS1 can be configured to show this. 28
    • working with directories 6.4. path completion The tab key can help you in typing a path without errors. Typing cd /et followed by the tab key will expand the command line to cd /etc/. When typing cd /Et followed by the tab key, nothing will happen because you typed the wrong path (upper case E). You will need fewer key strokes when using the tab key, and you will be sure your typed path is correct! 6.5. ls You can list the contents of a directory with ls. paul@pasha:~$ ls allfiles.txt dmesg.txt paul@pasha:~$ httpd.conf stuff summer.txt ls -a A frequently used option with ls is -a to show all files. Showing all files means including the hidden files. When a file name on a Unix file system starts with a dot, it is considered a hidden file and it doesn't show up in regular file listings. paul@pasha:~$ ls allfiles.txt dmesg.txt httpd.conf stuff summer.txt paul@pasha:~$ ls -a . allfiles.txt .bash_profile dmesg.txt .lesshst .. .bash_history .bashrc httpd.conf .ssh paul@pasha:~$ stuff summer.txt ls -l Many times you will be using options with ls to display the contents of the directory in different formats or to display different parts of the directory. Typing just ls gives you a list of files in the directory. Typing ls -l (that is a letter L, not the number 1) gives you a long listing. paul@pasha:~$ ls -l total 23992 -rw-r--r-- 1 paul paul 24506857 2006-03-30 22:53 allfiles.txt -rw-r--r-- 1 paul paul 14744 2006-09-27 11:45 dmesg.txt -rw-r--r-- 1 paul paul 8189 2006-03-31 14:01 httpd.conf drwxr-xr-x 2 paul paul 4096 2007-01-08 12:22 stuff -rw-r--r-- 1 paul paul 0 2006-03-30 22:45 summer.txt 29
    • working with directories ls -lh Another frequently used ls option is -h. It shows the numbers (file sizes) in a more human readable format. Also shown below is some variation in the way you can give the options to ls. We will explain the details of the output later in this book. paul@pasha:~$ ls -l -h total 24M -rw-r--r-- 1 paul paul -rw-r--r-- 1 paul paul -rw-r--r-- 1 paul paul drwxr-xr-x 2 paul paul -rw-r--r-- 1 paul paul paul@pasha:~$ ls -lh total 24M -rw-r--r-- 1 paul paul -rw-r--r-- 1 paul paul -rw-r--r-- 1 paul paul drwxr-xr-x 2 paul paul -rw-r--r-- 1 paul paul paul@pasha:~$ ls -hl total 24M -rw-r--r-- 1 paul paul -rw-r--r-- 1 paul paul -rw-r--r-- 1 paul paul drwxr-xr-x 2 paul paul -rw-r--r-- 1 paul paul paul@pasha:~$ ls -h -l total 24M -rw-r--r-- 1 paul paul -rw-r--r-- 1 paul paul -rw-r--r-- 1 paul paul drwxr-xr-x 2 paul paul -rw-r--r-- 1 paul paul 24M 15K 8.0K 4.0K 0 2006-03-30 2006-09-27 2006-03-31 2007-01-08 2006-03-30 22:53 11:45 14:01 12:22 22:45 allfiles.txt dmesg.txt httpd.conf stuff summer.txt 24M 15K 8.0K 4.0K 0 2006-03-30 2006-09-27 2006-03-31 2007-01-08 2006-03-30 22:53 11:45 14:01 12:22 22:45 allfiles.txt dmesg.txt httpd.conf stuff summer.txt 24M 15K 8.0K 4.0K 0 2006-03-30 2006-09-27 2006-03-31 2007-01-08 2006-03-30 22:53 11:45 14:01 12:22 22:45 allfiles.txt dmesg.txt httpd.conf stuff summer.txt 24M 15K 8.0K 4.0K 0 2006-03-30 2006-09-27 2006-03-31 2007-01-08 2006-03-30 22:53 11:45 14:01 12:22 22:45 allfiles.txt dmesg.txt httpd.conf stuff summer.txt 30
    • working with directories 6.6. mkdir Walking around the Unix file tree is fun, but it is even more fun to create your own directories with mkdir. You have to give at least one parameter to mkdir, the name of the new directory to be created. Think before you type a leading / . paul@laika:~$ mkdir MyDir paul@laika:~$ cd MyDir paul@laika:~/MyDir$ ls -al total 8 drwxr-xr-x 2 paul paul 4096 2007-01-10 21:13 . drwxr-xr-x 39 paul paul 4096 2007-01-10 21:13 .. paul@laika:~/MyDir$ mkdir stuff paul@laika:~/MyDir$ mkdir otherstuff paul@laika:~/MyDir$ ls -l total 8 drwxr-xr-x 2 paul paul 4096 2007-01-10 21:14 otherstuff drwxr-xr-x 2 paul paul 4096 2007-01-10 21:14 stuff paul@laika:~/MyDir$ mkdir -p When given the option -p, then mkdir will create parent directories as needed. paul@laika:~$ paul@laika:~$ MySubdir2 paul@laika:~$ ThreeDeep paul@laika:~$ mkdir -p MyDir2/MySubdir2/ThreeDeep ls MyDir2 ls MyDir2/MySubdir2 ls MyDir2/MySubdir2/ThreeDeep/ 6.7. rmdir When a directory is empty, you can use rmdir to remove the directory. paul@laika:~/MyDir$ rmdir otherstuff paul@laika:~/MyDir$ ls stuff paul@laika:~/MyDir$ cd .. paul@laika:~$ rmdir MyDir rmdir: MyDir/: Directory not empty paul@laika:~$ rmdir MyDir/stuff paul@laika:~$ rmdir MyDir rmdir -p And similar to the mkdir -p option, you can also use rmdir to recursively remove directories. paul@laika:~$ mkdir -p dir/subdir/subdir2 paul@laika:~$ rmdir -p dir/subdir/subdir2 paul@laika:~$ 31
    • working with directories 6.8. practice: working with directories 1. Display your current directory. 2. Change to the /etc directory. 3. Now change to your home directory using only three key presses. 4. Change to the /boot/grub directory using only eleven key presses. 5. Go to the parent directory of the current directory. 6. Go to the root directory. 7. List the contents of the root directory. 8. List a long listing of the root directory. 9. Stay where you are, and list the contents of /etc. 10. Stay where you are, and list the contents of /bin and /sbin. 11. Stay where you are, and list the contents of ~. 12. List all the files (including hidden files) in your home directory. 13. List the files in /boot in a human readable format. 14. Create a directory testdir in your home directory. 15. Change to the /etc directory, stay here and create a directory newdir in your home directory. 16. Create in one command the directories ~/dir1/dir2/dir3 (dir3 is a subdirectory from dir2, and dir2 is a subdirectory from dir1 ). 17. Remove the directory testdir. 18. If time permits (or if you are waiting for other students to finish this practice), use and understand pushd and popd. Use the man page of bash to find information about these commands. 32
    • working with directories 6.9. solution: working with directories 1. Display your current directory. pwd 2. Change to the /etc directory. cd /etc 3. Now change to your home directory using only three key presses. cd (and the enter key) 4. Change to the /boot/grub directory using only eleven key presses. cd /boot/grub (use the tab key) 5. Go to the parent directory of the current directory. cd .. (with space between cd and ..) 6. Go to the root directory. cd / 7. List the contents of the root directory. ls 8. List a long listing of the root directory. ls -l 9. Stay where you are, and list the contents of /etc. ls /etc 10. Stay where you are, and list the contents of /bin and /sbin. ls /bin /sbin 11. Stay where you are, and list the contents of ~. ls ~ 12. List all the files (including hidden files) in your home directory. ls -al ~ 13. List the files in /boot in a human readable format. ls -lh /boot 14. Create a directory testdir in your home directory. mkdir ~/testdir 15. Change to the /etc directory, stay here and create a directory newdir in your home directory. 33
    • working with directories cd /etc ; mkdir ~/newdir 16. Create in one command the directories ~/dir1/dir2/dir3 (dir3 is a subdirectory from dir2, and dir2 is a subdirectory from dir1 ). mkdir -p ~/dir1/dir2/dir3 17. Remove the directory testdir. rmdir testdir 18. If time permits (or if you are waiting for other students to finish this practice), use and understand pushd and popd. Use the man page of bash to find information about these commands. man bash The Bash shell has two built-in commands called pushd and popd. Both commands work with a common stack of previous directories. Pushd adds a directory to the stack and changes to a new current directory, popd removes a directory from the stack and sets the current directory. paul@laika:/etc$ cd /bin paul@laika:/bin$ pushd /lib /lib /bin paul@laika:/lib$ pushd /proc /proc /lib /bin paul@laika:/proc$ paul@laika:/proc$ popd /lib /bin paul@laika:/lib$ paul@laika:/lib$ paul@laika:/lib$ popd /bin paul@laika:/bin$ 34
    • Chapter 7. working with files Table of Contents 7.1. all files are case sensitive .............................................................................. 7.2. everything is a file ......................................................................................... 7.3. file .................................................................................................................. 7.4. touch ............................................................................................................... 7.5. rm ................................................................................................................... 7.6. cp .................................................................................................................... 7.7. mv .................................................................................................................. 7.8. rename ............................................................................................................ 7.9. practice: working with files ........................................................................... 7.10. solution: working with files ......................................................................... 36 36 36 37 37 38 39 40 41 42 In this chapter we learn how to recognise, create, remove, copy and move files using commands like file, touch, rm, cp, mv and rename. 35
    • working with files 7.1. all files are case sensitive Linux is case sensitive, this means that FILE1 is different from file1, and /etc/hosts is different from /etc/Hosts (the latter one does not exist on a typical Linux computer). This screenshot shows the difference between two files, one with upper case W, the other with lower case w. paul@laika:~/Linux$ ls winter.txt Winter.txt paul@laika:~/Linux$ cat winter.txt It is cold. paul@laika:~/Linux$ cat Winter.txt It is very cold! 7.2. everything is a file A directory is a special kind of file, but it is still a (case sensitive!) file. Even a terminal window (/dev/pts/4) or a hard disk (/dev/sdb) is represented somewhere in the file system as a file. It will become clear throughout this course that everything on Linux is a file. 7.3. file The file utility determines the file type. Linux does not use extensions to determine the file type. Your editor does not care whether a file ends in .TXT or .DOC. As a system administrator, you should use the file command to determine the file type. Here are some examples on a typical Linux system. paul@laika:~$ file pic33.png pic33.png: PNG image data, 3840 x 1200, 8-bit/color RGBA, non-interlaced paul@laika:~$ file /etc/passwd /etc/passwd: ASCII text paul@laika:~$ file HelloWorld.c HelloWorld.c: ASCII C program text The file command uses a magic file that contains patterns to recognise file types. The magic file is located in /usr/share/file/magic. Type man 5 magic for more information. It is interesting to point out file -s for special files like those in /dev and /proc. root@debian6~# file /dev/sda /dev/sda: block special root@debian6~# file -s /dev/sda /dev/sda: x86 boot sector; partition 1: ID=0x83, active, starthead... root@debian6~# file /proc/cpuinfo /proc/cpuinfo: empty root@debian6~# file -s /proc/cpuinfo /proc/cpuinfo: ASCII C++ program text 36
    • working with files 7.4. touch One easy way to create a file is with touch. (We will see many other ways for creating files later in this book.) paul@laika:~/test$ touch paul@laika:~/test$ touch paul@laika:~/test$ touch paul@laika:~/test$ ls -l total 0 -rw-r--r-- 1 paul paul 0 -rw-r--r-- 1 paul paul 0 -rw-r--r-- 1 paul paul 0 file1 file2 file555 2007-01-10 21:40 file1 2007-01-10 21:40 file2 2007-01-10 21:40 file555 touch -t Of course, touch can do more than just create files. Can you determine what by looking at the next screenshot? If not, check the manual for touch. paul@laika:~/test$ touch paul@laika:~/test$ touch paul@laika:~/test$ ls -l total 0 -rw-r--r-- 1 paul paul 0 -rw-r--r-- 1 paul paul 0 -t 200505050000 SinkoDeMayo -t 130207111630 BigBattle 1302-07-11 16:30 BigBattle 2005-05-05 00:00 SinkoDeMayo 7.5. rm When you no longer need a file, use rm to remove it. Unlike some graphical user interfaces, the command line in general does not have a waste bin or trash can to recover files. When you use rm to remove a file, the file is gone. Therefore, be careful when removing files! paul@laika:~/test$ ls BigBattle SinkoDeMayo paul@laika:~/test$ rm BigBattle paul@laika:~/test$ ls SinkoDeMayo rm -i To prevent yourself from accidentally removing a file, you can type rm -i. paul@laika:~/Linux$ touch brel.txt paul@laika:~/Linux$ rm -i brel.txt rm: remove regular empty file `brel.txt'? y paul@laika:~/Linux$ 37
    • working with files rm -rf By default, rm -r will not remove non-empty directories. However rm accepts several options that will allow you to remove any directory. The rm -rf statement is famous because it will erase anything (providing that you have the permissions to do so). When you are logged on as root, be very careful with rm -rf (the f means force and the r means recursive) since being root implies that permissions don't apply to you. You can literally erase your entire file system by accident. paul@laika:~$ ls test SinkoDeMayo paul@laika:~$ rm test rm: cannot remove `test': Is a directory paul@laika:~$ rm -rf test paul@laika:~$ ls test ls: test: No such file or directory 7.6. cp To copy a file, use cp with a source and a target argument. If the target is a directory, then the source files are copied to that target directory. paul@laika:~/test$ touch FileA paul@laika:~/test$ ls FileA paul@laika:~/test$ cp FileA FileB paul@laika:~/test$ ls FileA FileB paul@laika:~/test$ mkdir MyDir paul@laika:~/test$ ls FileA FileB MyDir paul@laika:~/test$ cp FileA MyDir/ paul@laika:~/test$ ls MyDir/ FileA cp -r To copy complete directories, use cp -r (the -r option forces recursive copying of all files in all subdirectories). paul@laika:~/test$ ls FileA FileB MyDir paul@laika:~/test$ ls MyDir/ FileA paul@laika:~/test$ cp -r MyDir MyDirB paul@laika:~/test$ ls FileA FileB MyDir MyDirB paul@laika:~/test$ ls MyDirB FileA 38
    • working with files cp multiple files to directory You can also use cp to copy multiple files into a directory. In this case, the last argument (a.k.a. the target) must be a directory. cp file1 file2 dir1/file3 dir1/file55 dir2 cp -i To prevent cp from overwriting existing files, use the -i (for interactive) option. paul@laika:~/test$ cp fire water paul@laika:~/test$ cp -i fire water cp: overwrite `water'? no paul@laika:~/test$ cp -p To preserve permissions and time stamps from source files, use cp -p. paul@laika:~/perms$ cp file* cp paul@laika:~/perms$ cp -p file* cpp paul@laika:~/perms$ ll * -rwx------ 1 paul paul 0 2008-08-25 13:26 file33 -rwxr-x--- 1 paul paul 0 2008-08-25 13:26 file42 cp: total 0 -rwx------ 1 paul paul 0 2008-08-25 13:34 file33 -rwxr-x--- 1 paul paul 0 2008-08-25 13:34 file42 cpp: total 0 -rwx------ 1 paul paul 0 2008-08-25 13:26 file33 -rwxr-x--- 1 paul paul 0 2008-08-25 13:26 file42 7.7. mv Use mv to rename a file or to move the file to another directory. paul@laika:~/test$ paul@laika:~/test$ file100 paul@laika:~/test$ paul@laika:~/test$ ABC.txt paul@laika:~/test$ touch file100 ls mv file100 ABC.txt ls When you need to rename only one file then mv is the preferred command to use. 39
    • working with files 7.8. rename The rename command can also be used but it has a more complex syntax to enable renaming of many files at once. Below are two examples, the first switches all occurrences of txt to png for all file names ending in .txt. The second example switches all occurrences of upper case ABC in lower case abc for all file names ending in .png . The following syntax will work on debian and ubuntu (prior to Ubuntu 7.10). paul@laika:~/test$ 123.txt ABC.txt paul@laika:~/test$ paul@laika:~/test$ 123.png ABC.png paul@laika:~/test$ paul@laika:~/test$ 123.png abc.png paul@laika:~/test$ ls rename 's/txt/png/' *.txt ls rename 's/ABC/abc/' *.png ls On Red Hat Enterprise Linux (and many other Linux distributions like Ubuntu 8.04), the syntax of rename is a bit different. The first example below renames all *.conf files replacing any occurrence of conf with bak. The second example renames all (*) files replacing one with ONE. [paul@RHEL4a test]$ one.conf two.conf [paul@RHEL4a test]$ [paul@RHEL4a test]$ one.bak two.bak [paul@RHEL4a test]$ [paul@RHEL4a test]$ ONE.bak two.bak [paul@RHEL4a test]$ ls rename conf bak *.conf ls rename one ONE * ls 40
    • working with files 7.9. practice: working with files 1. List the files in the /bin directory 2. Display the type of file of /bin/cat, /etc/passwd and /usr/bin/passwd. 3a. Download wolf.jpg and LinuxFun.pdf from http://linux-training.be (wget http:// linux-training.be/files/studentfiles/wolf.jpg and wget http://linux-training.be/files/ books/LinuxFun.pdf) 3b. Display the type of file of wolf.jpg and LinuxFun.pdf 3c. Rename wolf.jpg to wolf.pdf (use mv). 3d. Display the type of file of wolf.pdf and LinuxFun.pdf. 4. Create a directory ~/touched and enter it. 5. Create the files today.txt and yesterday.txt in touched. 6. Change the date on yesterday.txt to match yesterday's date. 7. Copy yesterday.txt to copy.yesterday.txt 8. Rename copy.yesterday.txt to kim 9. Create a directory called ~/testbackup and copy all files from ~/touched into it. 10. Use one command to remove the directory ~/testbackup and all files into it. 11. Create a directory ~/etcbackup and copy all *.conf files from /etc into it. Did you include all subdirectories of /etc ? 12. Use rename to rename all *.conf files to *.backup . (if you have more than one distro available, try it on all!) 41
    • working with files 7.10. solution: working with files 1. List the files in the /bin directory ls /bin 2. Display the type of file of /bin/cat, /etc/passwd and /usr/bin/passwd. file /bin/cat /etc/passwd /usr/bin/passwd 3a. Download wolf.jpg and LinuxFun.pdf from http://linux-training.be (wget http:// linux-training.be/files/studentfiles/wolf.jpg and wget http://linux-training.be/files/ books/LinuxFun.pdf) wget http://linux-training.be/files/studentfiles/wolf.jpg wget http://linux-training.be/files/studentfiles/wolf.png wget http://linux-training.be/files/books/LinuxFun.pdf 3b. Display the type of file of wolf.jpg and LinuxFun.pdf file wolf.jpg LinuxFun.pdf 3c. Rename wolf.jpg to wolf.pdf (use mv). mv wolf.jpg wolf.pdf 3d. Display the type of file of wolf.pdf and LinuxFun.pdf. file wolf.pdf LinuxFun.pdf 4. Create a directory ~/touched and enter it. mkdir ~/touched ; cd ~/touched 5. Create the files today.txt and yesterday.txt in touched. touch today.txt yesterday.txt 6. Change the date on yesterday.txt to match yesterday's date. touch -t 200810251405 yesterday.txt (substitute 20081025 with yesterday) 7. Copy yesterday.txt to copy.yesterday.txt cp yesterday.txt copy.yesterday.txt 8. Rename copy.yesterday.txt to kim mv copy.yesterday.txt kim 9. Create a directory called ~/testbackup and copy all files from ~/touched into it. mkdir ~/testbackup ; cp -r ~/touched ~/testbackup/ 10. Use one command to remove the directory ~/testbackup and all files into it. rm -rf ~/testbackup 11. Create a directory ~/etcbackup and copy all *.conf files from /etc into it. Did you include all subdirectories of /etc ? 42
    • working with files cp -r /etc/*.conf ~/etcbackup Only *.conf files that are directly in /etc/ are copied. 12. Use rename to rename all *.conf files to *.backup . (if you have more than one distro available, try it on all!) On RHEL: touch 1.conf 2.conf ; rename conf backup *.conf On Debian: touch 1.conf 2.conf ; rename 's/conf/backup/' *.conf 43
    • Chapter 8. working with file contents Table of Contents 8.1. 8.2. 8.3. 8.4. 8.5. 8.6. 8.7. 8.8. head ................................................................................................................ tail .................................................................................................................. cat ................................................................................................................... tac ................................................................................................................... more and less ................................................................................................. strings ............................................................................................................. practice: file contents ..................................................................................... solution: file contents ..................................................................................... 45 45 46 47 48 48 49 50 In this chapter we will look at the contents of text files with head, tail, cat, tac, more, less and strings. We will also get a glimpse of the possibilities of tools like cat on the command line. 44
    • working with file contents 8.1. head You can use head to display the first ten lines of a file. paul@laika:~$ head /etc/passwd root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/bin/sh bin:x:2:2:bin:/bin:/bin/sh sys:x:3:3:sys:/dev:/bin/sh sync:x:4:65534:sync:/bin:/bin/sync games:x:5:60:games:/usr/games:/bin/sh man:x:6:12:man:/var/cache/man:/bin/sh lp:x:7:7:lp:/var/spool/lpd:/bin/sh mail:x:8:8:mail:/var/mail:/bin/sh news:x:9:9:news:/var/spool/news:/bin/sh paul@laika:~$ The head command can also display the first n lines of a file. paul@laika:~$ head -4 /etc/passwd root:x:0:0:root:/root:/bin/bash daemon:x:1:1:daemon:/usr/sbin:/bin/sh bin:x:2:2:bin:/bin:/bin/sh sys:x:3:3:sys:/dev:/bin/sh Head can also display the first n bytes. paul@laika:~$ head -c4 /etc/passwd rootpaul@laika:~$ 8.2. tail Similar to head, the tail command will display the last ten lines of a file. paul@laika:~$ tail /etc/services vboxd 20012/udp binkp 24554/tcp asp 27374/tcp asp 27374/udp csync2 30865/tcp dircproxy 57000/tcp tfido 60177/tcp fido 60179/tcp # binkp fidonet protocol # Address Search Protocol # # # # cluster synchronization tool Detachable IRC Proxy fidonet EMSI over telnet fidonet EMSI over TCP # Local services paul@laika:~$ You can give tail the number of lines you want to see. $ tail -3 count.txt six seven eight The tail command has other useful options, some of which we will use during this course. 45
    • working with file contents 8.3. cat The cat command is one of the most universal tools. All it does is copy standard input to standard output. In combination with the shell this can be very powerful and diverse. Some examples will give a glimpse into the possibilities. The first example is simple, you can use cat to display a file on the screen. If the file is longer than the screen, it will scroll to the end. paul@laika:~$ cat /etc/resolv.conf nameserver 194.7.1.4 paul@laika:~$ concatenate cat is short for concatenate. One of the basic uses of cat is to concatenate files into a bigger (or complete) file. paul@laika:~$ paul@laika:~$ paul@laika:~$ paul@laika:~$ one two three paul@laika:~$ echo one > part1 echo two > part2 echo three > part3 cat part1 part2 part3 create files You can use cat to create flat text files. Type the cat > winter.txt command as shown in the screenshot below. Then type one or more lines, finishing each line with the enter key. After the last line, type and hold the Control (Ctrl) key and press d. paul@laika:~/test$ cat > winter.txt It is very cold today! paul@laika:~/test$ cat winter.txt It is very cold today! paul@laika:~/test$ The Ctrl d key combination will send an EOF (End of File) to the running process ending the cat command. 46
    • working with file contents custom end marker You can choose an end marker for cat with << as is shown in this screenshot. This construction is called a here directive and will end the cat command. paul@laika:~/test$ cat > hot.txt <<stop > It is hot today! > Yes it is summer. > stop paul@laika:~/test$ cat hot.txt It is hot today! Yes it is summer. paul@laika:~/test$ copy files In the third example you will see that cat can be used to copy files. We will explain in detail what happens here in the bash shell chapter. paul@laika:~/test$ cat winter.txt It is very cold today! paul@laika:~/test$ cat winter.txt > cold.txt paul@laika:~/test$ cat cold.txt It is very cold today! paul@laika:~/test$ 8.4. tac Just one example will show you the purpose of tac (as the opposite of cat). paul@laika:~/test$ cat count one two three four paul@laika:~/test$ tac count four three two one paul@laika:~/test$ 47
    • working with file contents 8.5. more and less The more command is useful for displaying files that take up more than one screen. More will allow you to see the contents of the file page by page. Use the space bar to see the next page, or q to quit. Some people prefer the less command to more. 8.6. strings With the strings command you can display readable ascii strings found in (binary) files. This example locates the ls binary then displays readable strings in the binary file (output is truncated). paul@laika:~$ which ls /bin/ls paul@laika:~$ strings /bin/ls /lib/ld-linux.so.2 librt.so.1 __gmon_start__ _Jv_RegisterClasses clock_gettime libacl.so.1 ... 48
    • working with file contents 8.7. practice: file contents 1. Display the first 12 lines of /etc/services. 2. Display the last line of /etc/passwd. 3. Use cat to create a file named count.txt that looks like this: One Two Three Four Five 4. Use cp to make a backup of this file to cnt.txt. 5. Use cat to make a backup of this file to catcnt.txt. 6. Display catcnt.txt, but with all lines in reverse order (the last line first). 7. Use more to display /var/log/messages. 8. Display the readable character strings from the /usr/bin/passwd command. 9. Use ls to find the biggest file in /etc. 10. Open two terminal windows (or tabs) and make sure you are in the same directory in both. Type echo this is the first line > tailing.txt in the first terminal, then issue tail -f tailing.txt in the second terminal. Now go back to the first terminal and type echo This is another line >> tailing.txt (note the double >>), verify that the tail -f in the second terminal shows both lines. Stop the tail -f with Ctrl-C. 11. Use cat to create a file named tailing.txt that contains the contents of tailing.txt followed by the contents of /etc/passwd. 12. Use cat to create a file named tailing.txt that contains the contents of tailing.txt preceded by the contents of /etc/passwd. 49
    • working with file contents 8.8. solution: file contents 1. Display the first 12 lines of /etc/services. head -12 /etc/services 2. Display the last line of /etc/passwd. tail -1 /etc/passwd 3. Use cat to create a file named count.txt that looks like this: cat > count.txt One Two Three Four Five (followed by Ctrl-d) 4. Use cp to make a backup of this file to cnt.txt. cp count.txt cnt.txt 5. Use cat to make a backup of this file to catcnt.txt. cat count.txt > catcnt.txt 6. Display catcnt.txt, but with all lines in reverse order (the last line first). tac catcnt.txt 7. Use more to display /var/log/messages. more /var/log/messages 8. Display the readable character strings from the /usr/bin/passwd command. strings /usr/bin/passwd 9. Use ls to find the biggest file in /etc. ls -lrS /etc 10. Open two terminal windows (or tabs) and make sure you are in the same directory in both. Type echo this is the first line > tailing.txt in the first terminal, then issue tail -f tailing.txt in the second terminal. Now go back to the first terminal and type echo This is another line >> tailing.txt (note the double >>), verify that the tail -f in the second terminal shows both lines. Stop the tail -f with Ctrl-C. 11. Use cat to create a file named tailing.txt that contains the contents of tailing.txt followed by the contents of /etc/passwd. cat /etc/passwd >> tailing.txt 12. Use cat to create a file named tailing.txt that contains the contents of tailing.txt preceded by the contents of /etc/passwd. mv tailing.txt tmp.txt ; cat /etc/passwd tmp.txt > tailing.txt 50
    • Chapter 9. the Linux file tree Table of Contents 9.1. filesystem hierarchy standard ........................................................................ 52 9.2. man hier ......................................................................................................... 52 9.3. the root directory / ......................................................................................... 52 9.4. binary directories ........................................................................................... 53 9.5. configuration directories ................................................................................ 55 9.6. data directories ............................................................................................... 57 9.7. in memory directories .................................................................................... 59 9.8. /usr Unix System Resources .......................................................................... 64 9.9. /var variable data ............................................................................................ 66 9.10. practice: file system tree .............................................................................. 68 9.11. solution: file system tree .............................................................................. 70 This chapter takes a look at the most common directories in the Linux file tree. It also shows that on Unix everything is a file. 51
    • the Linux file tree 9.1. filesystem hierarchy standard Many Linux distributions partially follow the Filesystem Hierarchy Standard. The FHS may help make more Unix/Linux file system trees conform better in the future. The FHS is available online at http://www.pathname.com/fhs/ where we read: "The filesystem hierarchy standard has been designed to be used by Unix distribution developers, package developers, and system implementers. However, it is primarily intended to be a reference and is not a tutorial on how to manage a Unix filesystem or directory hierarchy." 9.2. man hier There are some differences in the filesystems between Linux distributions. For help about your machine, enter man hier to find information about the file system hierarchy. This manual will explain the directory structure on your computer. 9.3. the root directory / All Linux systems have a directory structure that starts at the root directory. The root directory is represented by a forward slash, like this: /. Everything that exists on your Linux system can be found below this root directory. Let's take a brief look at the contents of the root directory. [paul@RHELv4u3 ~]$ ls / bin dev home media mnt boot etc lib misc opt proc root 52 sbin selinux srv sys tftpboot tmp usr var
    • the Linux file tree 9.4. binary directories Binaries are files that contain compiled source code (or machine code). Binaries can be executed on the computer. Sometimes binaries are called executables. /bin The /bin directory contains binaries for use by all users. According to the FHS the / bin directory should contain /bin/cat and /bin/date (among others). In the screenshot below you see common Unix/Linux commands like cat, cp, cpio, date, dd, echo, grep, and so on. Many of these will be covered in this book. paul@laika:~$ ls /bin archdetect egrep autopartition false bash fgconsole bunzip2 fgrep bzcat fuser bzcmp fusermount bzdiff get_mountoptions bzegrep grep bzexe gunzip bzfgrep gzexe bzgrep gzip bzip2 hostname bzip2recover hw-detect bzless ip bzmore kbd_mode cat kill ... mt mt-gnu mv nano nc nc.traditional netcat netstat ntfs-3g ntfs-3g.probe parted_devices parted_server partman partman-commit perform_recipe pidof setupcon sh sh.distrib sleep stralign stty su sync sysfs tailf tar tempfile touch true ulockmgr umount other /bin directories You can find a /bin subdirectory in many other directories. A user named serena could put her own programs in /home/serena/bin. Some applications, often when installed directly from source will put themselves in /opt. A samba server installation can use /opt/samba/bin to store its binaries. /sbin /sbin contains binaries to configure the operating system. Many of the system binaries require root privilege to perform certain tasks. Below a screenshot containing system binaries to change the ip address, partition a disk and create an ext4 file system. paul@ubu1010:~$ ls -l /sbin/ifconfig /sbin/fdisk /sbin/mkfs.ext4 -rwxr-xr-x 1 root root 97172 2011-02-02 09:56 /sbin/fdisk -rwxr-xr-x 1 root root 65708 2010-07-02 09:27 /sbin/ifconfig -rwxr-xr-x 5 root root 55140 2010-08-18 18:01 /sbin/mkfs.ext4 53
    • the Linux file tree /lib Binaries found in /bin and /sbin often use shared libraries located in /lib. Below is a screenshot of the partial contents of /lib. paul@laika:~$ ls /lib/libc* /lib/libc-2.5.so /lib/libcfont.so.0.0.0 /lib/libcap.so.1 /lib/libcidn-2.5.so /lib/libcap.so.1.10 /lib/libcidn.so.1 /lib/libcfont.so.0 /lib/libcom_err.so.2 /lib/libcom_err.so.2.1 /lib/libconsole.so.0 /lib/libconsole.so.0.0.0 /lib/libcrypt-2.5.so /lib/modules Typically, the Linux kernel loads kernel modules from /lib/modules/$kernelversion/. This directory is discussed in detail in the Linux kernel chapter. /lib32 and /lib64 We currently are in a transition between 32-bit and 64-bit systems. Therefore, you may encounter directories named /lib32 and /lib64 which clarify the register size used during compilation time of the libraries. A 64-bit computer may have some 32-bit binaries and libraries for compatibility with legacy applications. This screenshot uses the file utility to demonstrate the difference. paul@laika:~$ file /lib32/libc-2.5.so /lib32/libc-2.5.so: ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), for GNU/Linux 2.6.0, stripped paul@laika:~$ file /lib64/libcap.so.1.10 /lib64/libcap.so.1.10: ELF 64-bit LSB shared object, AMD x86-64, version 1 (SYSV), stripped The ELF (Executable and Linkable Format) is used in almost every Unix-like operating system since System V. /opt The purpose of /opt is to store optional software. In many cases this is software from outside the distribution repository. You may find an empty /opt directory on many systems. A large package can install all its files in /bin, /lib, /etc subdirectories within /opt/ $packagename/. If for example the package is called wp, then it installs in /opt/wp, putting binaries in /opt/wp/bin and manpages in /opt/wp/man. 54
    • the Linux file tree 9.5. configuration directories /boot The /boot directory contains all files needed to boot the computer. These files don't change very often. On Linux systems you typically find the /boot/grub directory here. /boot/grub contains /boot/grub/grub.cfg (older systems may still have /boot/ grub/grub.conf) which defines the boot menu that is displayed before the kernel starts. /etc All of the machine-specific configuration files should be located in /etc. Historically /etc stood for etcetera, today people often use the Editable Text Configuration backronym. Many times the name of a configuration files is the same as the application, daemon, or protocol with .conf added as the extension. paul@laika:~$ ls /etc/*.conf /etc/adduser.conf /etc/ld.so.conf /etc/brltty.conf /etc/lftp.conf /etc/ccertificates.conf /etc/libao.conf /etc/cvs-cron.conf /etc/logrotate.conf /etc/ddclient.conf /etc/ltrace.conf /etc/debconf.conf /etc/mke2fs.conf /etc/deluser.conf /etc/netscsid.conf /etc/fdmount.conf /etc/nsswitch.conf /etc/hdparm.conf /etc/pam.conf /etc/host.conf /etc/pnm2ppa.conf /etc/inetd.conf /etc/povray.conf /etc/kernel-img.conf /etc/resolv.conf paul@laika:~$ /etc/scrollkeeper.conf /etc/sysctl.conf /etc/syslog.conf /etc/ucf.conf /etc/uniconf.conf /etc/updatedb.conf /etc/usplash.conf /etc/uswsusp.conf /etc/vnc.conf /etc/wodim.conf /etc/wvdial.conf There is much more to be found in /etc. /etc/init.d/ A lot of Unix/Linux distributions have an /etc/init.d directory that contains scripts to start and stop daemons. This directory could disappear as Linux migrates to systems that replace the old init way of starting all daemons. /etc/X11/ The graphical display (aka X Window System or just X) is driven by software from the X.org foundation. The configuration file for your graphical display is /etc/X11/ xorg.conf. 55
    • the Linux file tree /etc/skel/ The skeleton directory /etc/skel is copied to the home directory of a newly created user. It usually contains hidden files like a .bashrc script. /etc/sysconfig/ This directory, which is not mentioned in the FHS, contains a lot of Red Hat Enterprise Linux configuration files. We will discuss some of them in greater detail. The screenshot below is the /etc/sysconfig directory from RHELv4u4 with everything installed. paul@RHELv4u4:~$ ls /etc/sysconfig/ apmd firstboot irda apm-scripts grub irqbalance authconfig hidd keyboard autofs httpd kudzu bluetooth hwconf lm_sensors clock i18n mouse console init mouse.B crond installinfo named desktop ipmi netdump diskdump iptables netdump_id_dsa dund iptables-cfg netdump_id_dsa.p paul@RHELv4u4:~$ network networking ntpd openib.conf pand pcmcia pgsql prelink rawdevices rhn samba saslauthd selinux spamassassin squid syslog sys-config-sec sys-config-users sys-logviewer tux vncservers xinetd The file /etc/sysconfig/firstboot tells the Red Hat Setup Agent not to run at boot time. If you want to run the Red Hat Setup Agent at the next reboot, then simply remove this file, and run chkconfig --level 5 firstboot on. The Red Hat Setup Agent allows you to install the latest updates, create a user account, join the Red Hat Network and more. It will then create the /etc/sysconfig/firstboot file again. paul@RHELv4u4:~$ cat /etc/sysconfig/firstboot RUN_FIRSTBOOT=NO The /etc/sysconfig/harddisks file contains some parameters to tune the hard disks. The file explains itself. You can see hardware detected by kudzu in /etc/sysconfig/hwconf. Kudzu is software from Red Hat for automatic discovery and configuration of hardware. The keyboard type and keymap table are set in the /etc/sysconfig/keyboard file. For more console keyboard information, check the manual pages of keymaps(5), dumpkeys(1), loadkeys(1) and the directory /lib/kbd/keymaps/. root@RHELv4u4:/etc/sysconfig# cat keyboard KEYBOARDTYPE="pc" KEYTABLE="us" We will discuss networking files in this directory in the networking chapter. 56
    • the Linux file tree 9.6. data directories /home Users can store personal or project data under /home. It is common (but not mandatory by the fhs) practice to name the users home directory after the user name in the format /home/$USERNAME. For example: paul@ubu606:~$ ls /home geert annik sandra paul tom Besides giving every user (or every project or group) a location to store personal files, the home directory of a user also serves as a location to store the user profile. A typical Unix user profile contains many hidden files (files whose file name starts with a dot). The hidden files of the Unix user profiles contain settings specific for that user. paul@ubu606:~$ ls -d /home/paul/.* /home/paul/. /home/paul/.bash_profile /home/paul/.. /home/paul/.bashrc /home/paul/.bash_history /home/paul/.lesshst /home/paul/.ssh /home/paul/.viminfo /root On many systems /root is the default location for personal data and profile of the root user. If it does not exist by default, then some administrators create it. /srv You may use /srv for data that is served by your system. The FHS allows locating cvs, rsync, ftp and www data in this location. The FHS also approves administrative naming in /srv, like /srv/project55/ftp and /srv/sales/www. On Sun Solaris (or Oracle Solaris) /export is used for this purpose. /media The /media directory serves as a mount point for removable media devices such as CD-ROM's, digital cameras, and various usb-attached devices. Since /media is rather new in the Unix world, you could very well encounter systems running without this directory. Solaris 9 does not have it, Solaris 10 does. Most Linux distributions today mount all removable media in /media. paul@debian5:~$ ls /media/ cdrom cdrom0 usbdisk 57
    • the Linux file tree /mnt The /mnt directory should be empty and should only be used for temporary mount points (according to the FHS). Unix and Linux administrators used to create many directories here, like /mnt/ something/. You likely will encounter many systems with more than one directory created and/or mounted inside /mnt to be used for various local and remote filesystems. /tmp Applications and users should use /tmp to store temporary data when needed. Data stored in /tmp may use either disk space or RAM. Both of which are managed by the operating system. Never use /tmp to store data that is important or which you wish to archive. 58
    • the Linux file tree 9.7. in memory directories /dev Device files in /dev appear to be ordinary files, but are not actually located on the hard disk. The /dev directory is populated with files as the kernel is recognising hardware. common physical devices Common hardware such as hard disk devices are represented by device files in /dev. Below a screenshot of SATA device files on a laptop and then IDE attached drives on a desktop. (The detailed meaning of these devices will be discussed later.) # # SATA or SCSI or USB # paul@laika:~$ ls /dev/sd* /dev/sda /dev/sda1 /dev/sda2 /dev/sda3 /dev/sdb /dev/sdb1 /dev/sdb2 # # IDE or ATAPI # paul@barry:~$ ls /dev/hd* /dev/hda /dev/hda1 /dev/hda2 /dev/hdb /dev/hdb1 /dev/hdb2 /dev/hdc Besides representing physical hardware, some device files are special. These special devices can be very useful. /dev/tty and /dev/pts For example, /dev/tty1 represents a terminal or console attached to the system. (Don't break your head on the exact terminology of 'terminal' or 'console', what we mean here is a command line interface.) When typing commands in a terminal that is part of a graphical interface like Gnome or KDE, then your terminal will be represented as /dev/pts/1 (1 can be another number). /dev/null On Linux you will find other special devices such as /dev/null which can be considered a black hole; it has unlimited storage, but nothing can be retrieved from it. Technically speaking, anything written to /dev/null will be discarded. /dev/null can be useful to discard unwanted output from commands. /dev/null is not a good location to store your backups ;-). 59
    • the Linux file tree /proc conversation with the kernel /proc is another special directory, appearing to be ordinary files, but not taking up disk space. It is actually a view of the kernel, or better, what the kernel manages, and is a means to interact with it directly. /proc is a proc filesystem. paul@RHELv4u4:~$ mount -t proc none on /proc type proc (rw) When listing the /proc directory you will see many numbers (on any Unix) and some interesting files (on Linux) mul@laika:~$ ls /proc 1 2339 4724 5418 10175 2523 4729 5421 10211 2783 4741 5658 10239 2975 4873 5661 141 29775 4874 5665 15045 29792 4878 5927 1519 2997 4879 6 1548 3 4881 6032 1551 30228 4882 6033 1554 3069 5 6145 1557 31422 5073 6298 1606 3149 5147 6414 180 31507 5203 6418 181 3189 5206 6419 182 3193 5228 6420 18898 3246 5272 6421 19799 3248 5291 6422 19803 3253 5294 6423 19804 3372 5356 6424 1987 4 5370 6425 1989 42 5379 6426 2 45 5380 6430 20845 4542 5412 6450 221 46 5414 6551 2338 4704 5416 6568 6587 6596 6599 6638 6652 6719 6736 6737 6755 6762 6774 6816 6991 6993 6996 7157 7163 7164 7171 7175 7188 7189 7191 7192 7199 7201 7204 7206 7214 7216 7218 7223 7224 7227 7260 7267 7275 7282 7298 7319 7330 7345 7513 7525 7529 9964 acpi asound buddyinfo bus cmdline cpuinfo crypto devices diskstats dma driver execdomains fb filesystems fs ide interrupts iomem ioports irq kallsyms kcore key-users kmsg loadavg locks meminfo misc modules mounts mtrr net pagetypeinfo partitions sched_debug scsi self slabinfo stat swaps sys sysrq-trigger sysvipc timer_list timer_stats tty uptime version version_signature vmcore vmnet vmstat zoneinfo Let's investigate the file properties inside /proc. Looking at the date and time will display the current date and time showing the files are constantly updated (a view on the kernel). paul@RHELv4u4:~$ date Mon Jan 29 18:06:32 EST 2007 paul@RHELv4u4:~$ ls -al /proc/cpuinfo -r--r--r-- 1 root root 0 Jan 29 18:06 /proc/cpuinfo paul@RHELv4u4:~$ paul@RHELv4u4:~$ ...time passes... paul@RHELv4u4:~$ paul@RHELv4u4:~$ date Mon Jan 29 18:10:00 EST 2007 paul@RHELv4u4:~$ ls -al /proc/cpuinfo -r--r--r-- 1 root root 0 Jan 29 18:10 /proc/cpuinfo 60
    • the Linux file tree Most files in /proc are 0 bytes, yet they contain data--sometimes a lot of data. You can see this by executing cat on files like /proc/cpuinfo, which contains information about the CPU. paul@RHELv4u4:~$ file /proc/cpuinfo /proc/cpuinfo: empty paul@RHELv4u4:~$ cat /proc/cpuinfo processor : 0 vendor_id : AuthenticAMD cpu family : 15 model : 43 model name : AMD Athlon(tm) 64 X2 Dual Core Processor 4600+ stepping : 1 cpu MHz : 2398.628 cache size : 512 KB fdiv_bug : no hlt_bug : no f00f_bug : no coma_bug : no fpu : yes fpu_exception : yes cpuid level : 1 wp : yes flags : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge... bogomips : 4803.54 Just for fun, here is /proc/cpuinfo on a Sun Sunblade 1000... paul@pasha:~$ cat /proc/cpuinfo cpu : TI UltraSparc III (Cheetah) fpu : UltraSparc III integrated FPU promlib : Version 3 Revision 2 prom : 4.2.2 type : sun4u ncpus probed : 2 ncpus active : 2 Cpu0Bogo : 498.68 Cpu0ClkTck : 000000002cb41780 Cpu1Bogo : 498.68 Cpu1ClkTck : 000000002cb41780 MMU Type : Cheetah State: CPU0: online CPU1: online Most of the files in /proc are read only, some require root privileges, some files are writable, and many files in /proc/sys are writable. Let's discuss some of the files in / proc. 61
    • the Linux file tree /proc/interrupts On the x86 architecture, /proc/interrupts displays the interrupts. paul@RHELv4u4:~$ cat /proc/interrupts CPU0 0: 13876877 IO-APIC-edge timer 1: 15 IO-APIC-edge i8042 8: 1 IO-APIC-edge rtc 9: 0 IO-APIC-level acpi 12: 67 IO-APIC-edge i8042 14: 128 IO-APIC-edge ide0 15: 124320 IO-APIC-edge ide1 169: 111993 IO-APIC-level ioc0 177: 2428 IO-APIC-level eth0 NMI: 0 LOC: 13878037 ERR: 0 MIS: 0 On a machine with two CPU's, the file looks like this. paul@laika:~$ cat /proc/interrupts CPU0 CPU1 0: 860013 0 IO-APIC-edge 1: 4533 0 IO-APIC-edge 7: 0 0 IO-APIC-edge 8: 6588227 0 IO-APIC-edge 10: 2314 0 IO-APIC-fasteoi 12: 133 0 IO-APIC-edge 14: 0 0 IO-APIC-edge 15: 72269 0 IO-APIC-edge 18: 1 0 IO-APIC-fasteoi 19: 115036 0 IO-APIC-fasteoi 20: 126871 0 IO-APIC-fasteoi 21: 30204 0 IO-APIC-fasteoi 22: 1334 0 IO-APIC-fasteoi 24: 234739 0 IO-APIC-fasteoi NMI: 72 42 LOC: 860000 859994 ERR: 0 timer i8042 parport0 rtc acpi i8042 libata libata yenta eth0 libata, ohci1394 ehci_hcd:usb1, uhci_hcd:usb2 saa7133[0], saa7133[0] nvidia /proc/kcore The physical memory is represented in /proc/kcore. Do not try to cat this file, instead use a debugger. The size of /proc/kcore is the same as your physical memory, plus four bytes. paul@laika:~$ ls -lh /proc/kcore -r-------- 1 root root 2.0G 2007-01-30 08:57 /proc/kcore paul@laika:~$ 62
    • the Linux file tree /sys Linux 2.6 hot plugging The /sys directory was created for the Linux 2.6 kernel. Since 2.6, Linux uses sysfs to support usb and IEEE 1394 (FireWire) hot plug devices. See the manual pages of udev(8) (the successor of devfs) and hotplug(8) for more info (or visit http://linuxhotplug.sourceforge.net/ ). Basically the /sys directory contains kernel information about hardware. 63
    • the Linux file tree 9.8. /usr Unix System Resources Although /usr is pronounced like user, remember that it stands for Unix System Resources. The /usr hierarchy should contain shareable, read only data. Some people choose to mount /usr as read only. This can be done from its own partition or from a read only NFS share. /usr/bin The /usr/bin directory contains a lot of commands. paul@deb508:~$ ls /usr/bin | wc -l 1395 (On Solaris the /bin directory is a symbolic link to /usr/bin.) /usr/include The /usr/include directory contains general use include files for C. paul@ubu1010:~$ ls /usr/include/ aalib.h expat_config.h af_vfs.h expat_external.h aio.h expat.h AL fcntl.h aliases.h features.h ... math.h mcheck.h memory.h menu.h mntent.h search.h semaphore.h setjmp.h sgtty.h shadow.h /usr/lib The /usr/lib directory contains libraries that are not directly executed by users or scripts. paul@deb508:~$ ls /usr/lib | head -7 4Suite ao apt arj aspell avahi bonobo /usr/local The /usr/local directory can be used by an administrator to install software locally. paul@deb508:~$ ls /usr/local/ bin etc games include lib man paul@deb508:~$ du -sh /usr/local/ 128K /usr/local/ 64 sbin share src
    • the Linux file tree /usr/share The /usr/share directory contains architecture independent data. As you can see, this is a fairly large directory. paul@deb508:~$ ls /usr/share/ | wc -l 263 paul@deb508:~$ du -sh /usr/share/ 1.3G /usr/share/ This directory typically contains /usr/share/man for manual pages. paul@deb508:~$ ls /usr/share/man cs fr hu it.UTF-8 man2 man6 pl.ISO8859-2 sv de fr.ISO8859-1 id ja man3 man7 pl.UTF-8 tr es fr.UTF-8 it ko man4 man8 pt_BR zh_CN fi gl it.ISO8859-1 man1 man5 pl ru zh_TW And it contains /usr/share/games for all static game data (so no high-scores or play logs). paul@ubu1010:~$ ls /usr/share/games/ openttd wesnoth /usr/src The /usr/src directory is the recommended location for kernel source files. paul@deb508:~$ ls -l /usr/src/ total 12 drwxr-xr-x 4 root root 4096 2011-02-01 14:43 linux-headers-2.6.26-2-686 drwxr-xr-x 18 root root 4096 2011-02-01 14:43 linux-headers-2.6.26-2-common drwxr-xr-x 3 root root 4096 2009-10-28 16:01 linux-kbuild-2.6.26 65
    • the Linux file tree 9.9. /var variable data Files that are unpredictable in size, such as log, cache and spool files, should be located in /var. /var/log The /var/log directory serves as a central point to contain all log files. [paul@RHEL4b ~]$ ls /var/log acpid cron.2 maillog.2 amanda cron.3 maillog.3 anaconda.log cron.4 maillog.4 anaconda.syslog cups mailman anaconda.xlog dmesg messages audit exim messages.1 boot.log gdm messages.2 boot.log.1 httpd messages.3 boot.log.2 iiim messages.4 boot.log.3 iptraf mysqld.log boot.log.4 lastlog news canna mail pgsql cron maillog ppp cron.1 maillog.1 prelink.log quagga radius rpmpkgs rpmpkgs.1 rpmpkgs.2 rpmpkgs.3 rpmpkgs.4 sa samba scrollkeeper.log secure secure.1 secure.2 secure.3 secure.4 spooler spooler.1 spooler.2 spooler.3 spooler.4 squid uucp vbox vmware-tools-guestd wtmp wtmp.1 Xorg.0.log Xorg.0.log.old /var/log/messages A typical first file to check when troubleshooting on Red Hat (and derivatives) is the /var/log/messages file. By default this file will contain information on what just happened to the system. The file is called /var/log/syslog on Debian and Ubuntu. [root@RHEL4b ~]# tail /var/log/messages Jul 30 05:13:56 anacron: anacron startup succeeded Jul 30 05:13:56 atd: atd startup succeeded Jul 30 05:13:57 messagebus: messagebus startup succeeded Jul 30 05:13:57 cups-config-daemon: cups-config-daemon startup succeeded Jul 30 05:13:58 haldaemon: haldaemon startup succeeded Jul 30 05:14:00 fstab-sync[3560]: removed all generated mount points Jul 30 05:14:01 fstab-sync[3628]: added mount point /media/cdrom for... Jul 30 05:14:01 fstab-sync[3646]: added mount point /media/floppy for... Jul 30 05:16:46 sshd(pam_unix)[3662]: session opened for user paul by... Jul 30 06:06:37 su(pam_unix)[3904]: session opened for user root by paul /var/cache The /var/cache directory can contain cache data for several applications. paul@ubu1010:~$ ls /var/cache/ apt dictionaries-common binfmts flashplugin-installer cups fontconfig debconf fonts gdm hald jockey ldconfig 66 man pm-utils pppconfig samba software-center
    • the Linux file tree /var/spool The /var/spool directory typically contains spool directories for mail and cron, but also serves as a parent directory for other spool files (for example print spool files). /var/lib The /var/lib directory contains application state information. Red Hat Enterprise Linux for example keeps files pertaining to rpm in /var/lib/rpm/. /var/... /var also contains Process ID files in /var/run (soon to be replaced with /run) and temporary files that survive a reboot in /var/tmp and information about file locks in /var/lock. There will be more examples of /var usage further in this book. 67
    • the Linux file tree 9.10. practice: file system tree 1. Does the file /bin/cat exist ? What about /bin/dd and /bin/echo. What is the type of these files ? 2. What is the size of the Linux kernel file(s) (vmlinu*) in /boot ? 3. Create a directory ~/test. Then issue the following commands: cd ~/test dd if=/dev/zero of=zeroes.txt count=1 bs=100 od zeroes.txt dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file / dev/zero to ~/test/zeroes.txt. Can you describe the functionality of /dev/zero ? 4. Now issue the following command: dd if=/dev/random of=random.txt count=1 bs=100 ; od random.txt dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file / dev/random to ~/test/random.txt. Can you describe the functionality of /dev/random ? 5. Issue the following two commands, and look at the first character of each output line. ls -l /dev/sd* /dev/hd* ls -l /dev/tty* /dev/input/mou* The first ls will show block(b) devices, the second ls shows character(c) devices. Can you tell the difference between block and character devices ? 6. Use cat to display /etc/hosts and /etc/resolv.conf. What is your idea about the purpose of these files ? 7. Are there any files in /etc/skel/ ? Check also for hidden files. 8. Display /proc/cpuinfo. On what architecture is your Linux running ? 9. Display /proc/interrupts. What is the size of this file ? Where is this file stored ? 10. Can you enter the /root directory ? Are there (hidden) files ? 11. Are ifconfig, fdisk, parted, shutdown and grub-install present in /sbin ? Why are these binaries in /sbin and not in /bin ? 12. Is /var/log a file or a directory ? What about /var/spool ? 13. Open two command prompts (Ctrl-Shift-T in gnome-terminal) or terminals (CtrlAlt-F1, Ctrl-Alt-F2, ...) and issue the who am i in both. Then try to echo a word from one terminal to the other. 68
    • the Linux file tree 14. Read the man page of random and explain the difference between /dev/random and /dev/urandom. 69
    • the Linux file tree 9.11. solution: file system tree 1. Does the file /bin/cat exist ? What about /bin/dd and /bin/echo. What is the type of these files ? ls /bin/cat ; file /bin/cat ls /bin/dd ; file /bin/dd ls /bin/echo ; file /bin/echo 2. What is the size of the Linux kernel file(s) (vmlinu*) in /boot ? ls -lh /boot/vm* 3. Create a directory ~/test. Then issue the following commands: cd ~/test dd if=/dev/zero of=zeroes.txt count=1 bs=100 od zeroes.txt dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file / dev/zero to ~/test/zeroes.txt. Can you describe the functionality of /dev/zero ? /dev/zero is a Linux special device. It can be considered a source of zeroes. You cannot send something to /dev/zero, but you can read zeroes from it. 4. Now issue the following command: dd if=/dev/random of=random.txt count=1 bs=100 ; od random.txt dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file / dev/random to ~/test/random.txt. Can you describe the functionality of /dev/random ? /dev/random acts as a random number generator on your Linux machine. 5. Issue the following two commands, and look at the first character of each output line. ls -l /dev/sd* /dev/hd* ls -l /dev/tty* /dev/input/mou* The first ls will show block(b) devices, the second ls shows character(c) devices. Can you tell the difference between block and character devices ? Block devices are always written to (or read from) in blocks. For hard disks, blocks of 512 bytes are common. Character devices act as a stream of characters (or bytes). Mouse and keyboard are typical character devices. 6. Use cat to display /etc/hosts and /etc/resolv.conf. What is your idea about the purpose of these files ? 70
    • the Linux file tree /etc/hosts contains hostnames with their ip address /etc/resolv.conf should contain the ip address of a DNS name server. 7. Are there any files in /etc/skel/ ? Check also for hidden files. Issue "ls -al /etc/skel/". Yes, there should be hidden files there. 8. Display /proc/cpuinfo. On what architecture is your Linux running ? The file should contain at least one line with Intel or other cpu. 9. Display /proc/interrupts. What is the size of this file ? Where is this file stored ? The size is zero, yet the file contains data. It is not stored anywhere because /proc is a virtual file system that allows you to talk with the kernel. (If you answered "stored in RAM-memory, that is also correct...). 10. Can you enter the /root directory ? Are there (hidden) files ? Try "cd /root". Yes there are (hidden) files there. 11. Are ifconfig, fdisk, parted, shutdown and grub-install present in /sbin ? Why are these binaries in /sbin and not in /bin ? Because those files are only meant for system administrators. 12. Is /var/log a file or a directory ? What about /var/spool ? Both are directories. 13. Open two command prompts (Ctrl-Shift-T in gnome-terminal) or terminals (CtrlAlt-F1, Ctrl-Alt-F2, ...) and issue the who am i in both. Then try to echo a word from one terminal to the other. tty-terminal: echo Hello > /dev/tty1 pts-terminal: echo Hello > /dev/pts/1 14. Read the man page of random and explain the difference between /dev/random and /dev/urandom. man 4 random 71
    • Part III. shell expansion
    • Chapter 10. commands and arguments Table of Contents 10.1. 10.2. 10.3. 10.4. 10.5. 10.6. 10.7. echo .............................................................................................................. arguments ..................................................................................................... commands .................................................................................................... aliases ........................................................................................................... displaying shell expansion ........................................................................... practice: commands and arguments ............................................................. solution: commands and arguments ............................................................. 74 74 76 77 78 79 81 This chapter introduces you to shell expansion by taking a close look at commands and arguments. Knowing shell expansion is important because many commands on your Linux system are processed and most likely changed by the shell before they are executed. The command line interface or shell used on most Linux systems is called bash, which stands for Bourne again shell. The bash shell incorporates features from sh (the original Bourne shell), csh (the C shell), and ksh (the Korn shell). 73
    • commands and arguments 10.1. echo This chapter frequently uses the echo command to demonstrate shell features. The echo command is very simple: it echoes the input that it receives. paul@laika:~$ echo Burtonville Burtonville paul@laika:~$ echo Smurfs are blue Smurfs are blue 10.2. arguments One of the primary features of a shell is to perform a command line scan. When you enter a command at the shell's command prompt and press the enter key, then the shell will start scanning that line, cutting it up in arguments. While scanning the line, the shell may make many changes to the arguments you typed. This process is called shell expansion. When the shell has finished scanning and modifying that line, then it will be executed. white space removal Parts that are separated by one or more consecutive white spaces (or tabs) are considered separate arguments, any white space is removed. The first argument is the command to be executed, the other arguments are given to the command. The shell effectively cuts your command into one or more arguments. This explains why the following four different command lines are the same after shell expansion. [paul@RHELv4u3 Hello World [paul@RHELv4u3 Hello World [paul@RHELv4u3 Hello World [paul@RHELv4u3 Hello World ~]$ echo Hello World ~]$ echo Hello ~]$ echo ~]$ World echo Hello World Hello World The echo command will display each argument it receives from the shell. The echo command will also add a new white space between the arguments it received. single quotes You can prevent the removal of white spaces by quoting the spaces. The contents of the quoted string are considered as one argument. In the screenshot below the echo receives only one argument. [paul@RHEL4b ~]$ echo 'A line with A line with single quotes [paul@RHEL4b ~]$ 74 single quotes'
    • commands and arguments double quotes You can also prevent the removal of white spaces by double quoting the spaces. Same as above, echo only receives one argument. [paul@RHEL4b ~]$ echo "A line with A line with double quotes [paul@RHEL4b ~]$ double quotes" Later in this book, when discussing variables we will see important differences between single and double quotes. echo and quotes Quoted lines can include special escaped characters recognised by the echo command (when using echo -e). The screenshot below shows how to use n for a newline and t for a tab (usually eight white spaces). [paul@RHEL4b A line with a newline [paul@RHEL4b A line with a newline [paul@RHEL4b A line with [paul@RHEL4b A line with [paul@RHEL4b ~]$ echo -e "A line with na newline" ~]$ echo -e 'A line with na newline' ~]$ echo -e "A line with ta tab" a tab ~]$ echo -e 'A line with ta tab' a tab ~]$ The echo command can generate more than white spaces, tabs and newlines. Look in the man page for a list of options. 75
    • commands and arguments 10.3. commands external or builtin commands ? Not all commands are external to the shell, some are builtin. External commands are programs that have their own binary and reside somewhere in the file system. Many external commands are located in /bin or /sbin. Builtin commands are an integral part of the shell program itself. type To find out whether a command given to the shell will be executed as an external command or as a builtin command, use the type command. paul@laika:~$ type cd cd is a shell builtin paul@laika:~$ type cat cat is /bin/cat As you can see, the cd command is builtin and the cat command is external. You can also use this command to show you whether the command is aliased or not. paul@laika:~$ type ls ls is aliased to `ls --color=auto' running external commands Some commands have both builtin and external versions. When one of these commands is executed, the builtin version takes priority. To run the external version, you must enter the full path to the command. paul@laika:~$ type -a echo echo is a shell builtin echo is /bin/echo paul@laika:~$ /bin/echo Running the external echo command... Running the external echo command... which The which command will search for binaries in the $PATH environment variable (variables will be explained later). In the screenshot below, it is determined that cd is builtin, and ls, cp, rm, mv, mkdir, pwd, and which are external commands. [root@RHEL4b ~]# which cp ls cd mkdir pwd /bin/cp /bin/ls /usr/bin/which: no cd in (/usr/kerberos/sbin:/usr/kerberos/bin:... /bin/mkdir /bin/pwd 76
    • commands and arguments 10.4. aliases create an alias The shell allows you to create aliases. Aliases are often used to create an easier to remember name for an existing command or to easily supply parameters. [paul@RHELv4u3 ~]$ cat count.txt one two three [paul@RHELv4u3 ~]$ alias dog=tac [paul@RHELv4u3 ~]$ dog count.txt three two one abbreviate commands An alias can also be useful to abbreviate an existing command. paul@laika:~$ alias ll='ls -lh --color=auto' paul@laika:~$ alias c='clear' paul@laika:~$ default options Aliases can be used to supply commands with default options. The example below shows how to set the -i option default when typing rm. [paul@RHELv4u3 ~]$ rm: remove regular [paul@RHELv4u3 ~]$ [paul@RHELv4u3 ~]$ ls: winter.txt: No [paul@RHELv4u3 ~]$ [paul@RHELv4u3 ~]$ [paul@RHELv4u3 ~]$ rm: remove regular [paul@RHELv4u3 ~]$ rm -i winter.txt file `winter.txt'? no rm winter.txt ls winter.txt such file or directory touch winter.txt alias rm='rm -i' rm winter.txt empty file `winter.txt'? no Some distributions enable default aliases to protect users from accidentally erasing files ('rm -i', 'mv -i', 'cp -i') viewing aliases You can provide one or more aliases as arguments to the alias command to get their definitions. Providing no arguments gives a complete list of current aliases. paul@laika:~$ alias c ll alias c='clear' alias ll='ls -lh --color=auto' 77
    • commands and arguments unalias You can undo an alias with the unalias command. [paul@RHEL4b ~]$ /bin/rm [paul@RHEL4b ~]$ [paul@RHEL4b ~]$ alias rm='rm -i' /bin/rm [paul@RHEL4b ~]$ [paul@RHEL4b ~]$ /bin/rm [paul@RHEL4b ~]$ which rm alias rm='rm -i' which rm unalias rm which rm 10.5. displaying shell expansion You can display shell expansion with set -x, and stop displaying it with set +x. You might want to use this further on in this course, or when in doubt about exactly what the shell is doing with your command. [paul@RHELv4u3 ~]$ set -x ++ echo -ne '033]0;paul@RHELv4u3:~007' [paul@RHELv4u3 ~]$ echo $USER + echo paul paul ++ echo -ne '033]0;paul@RHELv4u3:~007' [paul@RHELv4u3 ~]$ echo $USER + echo '$USER' $USER ++ echo -ne '033]0;paul@RHELv4u3:~007' [paul@RHELv4u3 ~]$ set +x + set +x [paul@RHELv4u3 ~]$ echo $USER paul 78
    • commands and arguments 10.6. practice: commands and arguments 1. How many arguments are in this line (not counting the command itself). touch '/etc/cron/cron.allow' 'file 42.txt' "file 33.txt" 2. Is tac a shell builtin command ? 3. Is there an existing alias for rm ? 4. Read the man page of rm, make sure you understand the -i option of rm. Create and remove a file to test the -i option. 5. Execute: alias rm='rm -i' . Test your alias with a test file. Does this work as expected ? 6. List all current aliases. 7a. Create an alias called 'city' that echoes your hometown. 7b. Use your alias to test that it works. 8. Execute set -x to display shell expansion for every command. 9. Test the functionality of set -x by executing your city and rm aliases. 10 Execute set +x to stop displaying shell expansion. 11. Remove your city alias. 12. What is the location of the cat and the passwd commands ? 13. Explain the difference between the following commands: echo /bin/echo 14. Explain the difference between the following commands: echo Hello echo -n Hello 15. Display A B C with two spaces between B and C. (optional)16. Complete the following command (do not use spaces) to display exactly the following output: 4+4 10+14 =8 =24 18. Use echo to display the following exactly: ?? 79
    • commands and arguments Find two solutions with single quotes, two with double quotes and one without quotes (and say thank you to René and Darioush from Google for this extra). 19. Use one echo command to display three words on three lines. 80
    • commands and arguments 10.7. solution: commands and arguments 1. How many arguments are in this line (not counting the command itself). touch '/etc/cron/cron.allow' 'file 42.txt' "file 33.txt" answer: three 2. Is tac a shell builtin command ? type tac 3. Is there an existing alias for rm ? alias rm 4. Read the man page of rm, make sure you understand the -i option of rm. Create and remove a file to test the -i option. man rm touch testfile rm -i testfile 5. Execute: alias rm='rm -i' . Test your alias with a test file. Does this work as expected ? touch testfile rm testfile (should ask for confirmation) 6. List all current aliases. alias 7a. Create an alias called 'city' that echoes your hometown. alias city='echo Antwerp' 7b. Use your alias to test that it works. city (it should display Antwerp) 8. Execute set -x to display shell expansion for every command. set -x 9. Test the functionality of set -x by executing your city and rm aliases. shell should display the resolved aliases and then execute the command: paul@deb503:~$ set -x paul@deb503:~$ city + echo antwerp antwerp 10 Execute set +x to stop displaying shell expansion. set +x 11. Remove your city alias. 81
    • commands and arguments unalias city 12. What is the location of the cat and the passwd commands ? which cat (probably /bin/cat) which passwd (probably /usr/bin/passwd) 13. Explain the difference between the following commands: echo /bin/echo The echo command will be interpreted by the shell as the built-in echo command. The /bin/echo command will make the shell execute the echo binary located in the /bin directory. 14. Explain the difference between the following commands: echo Hello echo -n Hello The -n option of the echo command will prevent echo from echoing a trailing newline. echo Hello will echo six characters in total, echo -n hello only echoes five characters. (The -n option might not work in the Korn shell.) 15. Display A B C with two spaces between B and C. echo "A B C" 16. Complete the following command (do not use spaces) to display exactly the following output: 4+4 10+14 =8 =24 The solution is to use tabs with t. echo -e "4+4t=8" ; echo -e "10+14t=24" 18. Use echo to display the following exactly: ?? echo echo echo echo echo '??' -e '??' "??" -e "??" ?? Find two solutions with single quotes, two with double quotes and one without quotes (and say thank you to René and Darioush from Google for this extra). 19. Use one echo command to display three words on three lines. echo -e "one ntwo nthree" 82
    • Chapter 11. control operators Table of Contents 11.1. ; semicolon ................................................................................................... 11.2. & ampersand ................................................................................................ 11.3. $? dollar question mark ............................................................................... 11.4. && double ampersand ................................................................................. 11.5. || double vertical bar .................................................................................... 11.6. combining && and || ................................................................................... 11.7. # pound sign ................................................................................................ 11.8. escaping special characters ........................................................................ 11.9. practice: control operators ........................................................................... 11.10. solution: control operators ......................................................................... 84 84 84 85 85 85 86 86 87 88 In this chapter we put more than one command on the command line using control operators. We also briefly discuss related parameters ($?) and similar special characters(&). 83
    • control operators 11.1. ; semicolon You can put two or more commands on the same line separated by a semicolon ; . The shell will scan the line until it reaches the semicolon. All the arguments before this semicolon will be considered a separate command from all the arguments after the semicolon. Both series will be executed sequentially with the shell waiting for each command to finish before starting the next one. [paul@RHELv4u3 Hello [paul@RHELv4u3 World [paul@RHELv4u3 Hello World [paul@RHELv4u3 ~]$ echo Hello ~]$ echo World ~]$ echo Hello ; echo World ~]$ 11.2. & ampersand When a line ends with an ampersand &, the shell will not wait for the command to finish. You will get your shell prompt back, and the command is executed in background. You will get a message when this command has finished executing in background. [paul@RHELv4u3 ~]$ sleep 20 & [1] 7925 [paul@RHELv4u3 ~]$ ...wait 20 seconds... [paul@RHELv4u3 ~]$ [1]+ Done sleep 20 The technical explanation of what happens in this case is explained in the chapter about processes. 11.3. $? dollar question mark The exit code of the previous command is stored in the shell variable $?. Actually $? is a shell parameter and not a variable, since you cannot assign a value to $?. paul@debian5:~/test$ touch file1 paul@debian5:~/test$ echo $? 0 paul@debian5:~/test$ rm file1 paul@debian5:~/test$ echo $? 0 paul@debian5:~/test$ rm file1 rm: cannot remove `file1': No such file or directory paul@debian5:~/test$ echo $? 1 paul@debian5:~/test$ 84
    • control operators 11.4. && double ampersand The shell will interpret && as a logical AND. When using && the second command is executed only if the first one succeeds (returns a zero exit status). paul@barry:~$ echo first && echo second first second paul@barry:~$ zecho first && echo second -bash: zecho: command not found Another example of the same logical AND principle. This example starts with a working cd followed by ls, then a non-working cd which is not followed by ls. [paul@RHELv4u3 ~]$ cd gen && ls file1 file3 File55 fileab FileAB fileabc file2 File4 FileA Fileab fileab2 [paul@RHELv4u3 gen]$ cd gen && ls -bash: cd: gen: No such file or directory 11.5. || double vertical bar The || represents a logical OR. The second command is executed only when the first command fails (returns a non-zero exit status). paul@barry:~$ echo first || echo second ; echo third first third paul@barry:~$ zecho first || echo second ; echo third -bash: zecho: command not found second third paul@barry:~$ Another example of the same logical OR principle. [paul@RHELv4u3 ~]$ cd gen || ls [paul@RHELv4u3 gen]$ cd gen || ls -bash: cd: gen: No such file or directory file1 file3 File55 fileab FileAB fileabc file2 File4 FileA Fileab fileab2 11.6. combining && and || You can use this logical AND and logical OR to write an if-then-else structure on the command line. This example uses echo to display whether the rm command was successful. paul@laika:~/test$ rm file1 && echo It worked! || echo It failed! It worked! paul@laika:~/test$ rm file1 && echo It worked! || echo It failed! rm: cannot remove `file1': No such file or directory It failed! paul@laika:~/test$ 85
    • control operators 11.7. # pound sign Everything written after a pound sign (#) is ignored by the shell. This is useful to write a shell comment, but has no influence on the command execution or shell expansion. paul@debian4:~$ mkdir test paul@debian4:~$ cd test paul@debian4:~/test$ ls paul@debian4:~/test$ # we create a directory #### we enter the directory # is it empty ? 11.8. escaping special characters The backslash character enables the use of control characters, but without the shell interpreting it, this is called escaping characters. [paul@RHELv4u3 hello ; world [paul@RHELv4u3 hello world [paul@RHELv4u3 escaping # & [paul@RHELv4u3 escaping ?*"' ~]$ echo hello ; world ~]$ echo hello world ~]$ echo escaping # & " ' " ' ~]$ echo escaping ?*"' end of line backslash Lines ending in a backslash are continued on the next line. The shell does not interpret the newline character and will wait on shell expansion and execution of the command line until a newline without backslash is encountered. [paul@RHEL4b ~]$ echo This command line > is split in three > parts This command line is split in three parts [paul@RHEL4b ~]$ 86
    • control operators 11.9. practice: control operators 0. Each question can be answered by one command line! 1. When you type passwd, which file is executed ? 2. What kind of file is that ? 3. Execute the pwd command twice. (remember 0.) 4. Execute ls after cd /etc, but only if cd /etc did not error. 5. Execute cd /etc after cd etc, but only if cd etc fails. 6. Echo it worked when touch test42 works, and echo it failed when the touch failed. All on one command line as a normal user (not root). Test this line in your home directory and in /bin/ . 7. Execute sleep 6, what is this command doing ? 8. Execute sleep 200 in background (do not wait for it to finish). 9. Write a command line that executes rm file55. Your command line should print 'success' if file55 is removed, and print 'failed' if there was a problem. (optional)10. Use echo to display "Hello World with strange' characters * [ } ~ ." (including all quotes) 87
    • control operators 11.10. solution: control operators 0. Each question can be answered by one command line! 1. When you type passwd, which file is executed ? which passwd 2. What kind of file is that ? file /usr/bin/passwd 3. Execute the pwd command twice. (remember 0.) pwd ; pwd 4. Execute ls after cd /etc, but only if cd /etc did not error. cd /etc && ls 5. Execute cd /etc after cd etc, but only if cd etc fails. cd etc || cd /etc 6. Echo it worked when touch test42 works, and echo it failed when the touch failed. All on one command line as a normal user (not root). Test this line in your home directory and in /bin/ . paul@deb503:~$ cd ; touch test42 && echo it worked || echo it failed it worked paul@deb503:~$ cd /bin; touch test42 && echo it worked || echo it failed touch: cannot touch `test42': Permission denied it failed 7. Execute sleep 6, what is this command doing ? pausing for six seconds 8. Execute sleep 200 in background (do not wait for it to finish). sleep 200 & 9. Write a command line that executes rm file55. Your command line should print 'success' if file55 is removed, and print 'failed' if there was a problem. rm file55 && echo success || echo failed (optional)10. Use echo to display "Hello World with strange' characters * [ } ~ ." (including all quotes) echo "Hello World with strange' characters * [ } ~ . " or echo ""Hello World with strange' characters * [ } ~ . "" 88
    • Chapter 12. variables Table of Contents 12.1. about variables ............................................................................................. 12.2. quotes ........................................................................................................... 12.3. set ................................................................................................................. 12.4. unset ............................................................................................................. 12.5. env ................................................................................................................ 12.6. export ........................................................................................................... 12.7. delineate variables ........................................................................................ 12.8. unbound variables ........................................................................................ 12.9. shell options ................................................................................................. 12.10. shell embedding ......................................................................................... 12.11. practice: shell variables .............................................................................. 12.12. solution: shell variables ............................................................................. 90 92 92 92 93 93 94 94 95 96 97 98 In this chapter we learn to manage environment variables in the shell. These variables are often read by applications. We also take a brief look at child shells, embedded shells and shell options. 89
    • variables 12.1. about variables $ dollar sign Another important character interpreted by the shell is the dollar sign $. The shell will look for an environment variable named like the string following the dollar sign and replace it with the value of the variable (or with nothing if the variable does not exist). These are some examples using $HOSTNAME, $USER, $UID, $SHELL, and $HOME. [paul@RHELv4u3 ~]$ echo This is the $SHELL shell This is the /bin/bash shell [paul@RHELv4u3 ~]$ echo This is $SHELL on computer $HOSTNAME This is /bin/bash on computer RHELv4u3.localdomain [paul@RHELv4u3 ~]$ echo The userid of $USER is $UID The userid of paul is 500 [paul@RHELv4u3 ~]$ echo My homedir is $HOME My homedir is /home/paul case sensitive This example shows that shell variables are case sensitive! [paul@RHELv4u3 ~]$ echo Hello $USER Hello paul [paul@RHELv4u3 ~]$ echo Hello $user Hello $PS1 The $PS1 variable determines your shell prompt. You can use backslash escaped special characters like u for the username or w for the working directory. The bash manual has a complete reference. In this example we change the value of $PS1 a couple of times. paul@deb503:~$ PS1=prompt prompt promptPS1='prompt ' prompt prompt PS1='> ' > > PS1='u@h$ ' paul@deb503$ paul@deb503$ PS1='u@h:W$' paul@deb503:~$ 90
    • variables To avoid unrecoverable mistakes, you can set normal user prompts to green and the root prompt to red. Add the following to your .bashrc for a green user prompt: # color prompt by paul RED='[033[01;31m]' WHITE='[033[01;00m]' GREEN='[033[01;32m]' BLUE='[033[01;34m]' export PS1="${debian_chroot:+($debian_chroot)}$GREENu$WHITE@$BLUEh$WHITEw$ " $PATH The $PATH variable is determines where the shell is looking for commands to execute (unless the command is builtin or aliased). This variable contains a list of directories, separated by colons. [[paul@RHEL4b ~]$ echo $PATH /usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin: The shell will not look in the current directory for commands to execute! (Looking for executables in the current directory provided an easy way to hack PC-DOS computers). If you want the shell to look in the current directory, then add a . at the end of your $PATH. [paul@RHEL4b ~]$ PATH=$PATH:. [paul@RHEL4b ~]$ echo $PATH /usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:. [paul@RHEL4b ~]$ Your path might be different when using su instead of su - because the latter will take on the environment of the target user. The root user typically has /sbin directories added to the $PATH variable. [paul@RHEL3 ~]$ su Password: [root@RHEL3 paul]# echo $PATH /usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin [root@RHEL3 paul]# exit [paul@RHEL3 ~]$ su Password: [root@RHEL3 ~]# echo $PATH /usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin: [root@RHEL3 ~]# creating variables This example creates the variable $MyVar and sets its value. It then uses echo to verify the value. [paul@RHELv4u3 gen]$ MyVar=555 [paul@RHELv4u3 gen]$ echo $MyVar 555 [paul@RHELv4u3 gen]$ 91
    • variables 12.2. quotes Notice that double quotes still allow the parsing of variables, whereas single quotes prevent this. [paul@RHELv4u3 [paul@RHELv4u3 555 [paul@RHELv4u3 555 [paul@RHELv4u3 $MyVar ~]$ MyVar=555 ~]$ echo $MyVar ~]$ echo "$MyVar" ~]$ echo '$MyVar' The bash shell will replace variables with their value in double quoted lines, but not in single quoted lines. paul@laika:~$ city=Burtonville paul@laika:~$ echo "We are in $city today." We are in Burtonville today. paul@laika:~$ echo 'We are in $city today.' We are in $city today. 12.3. set You can use the set command to display a list of environment variables. On Ubuntu and Debian systems, the set command will also list shell functions after the shell variables. Use set | more to see the variables then. 12.4. unset Use the unset command to remove a variable from your shell environment. [paul@RHEL4b [paul@RHEL4b 8472 [paul@RHEL4b [paul@RHEL4b ~]$ MyVar=8472 ~]$ echo $MyVar ~]$ unset MyVar ~]$ echo $MyVar [paul@RHEL4b ~]$ 92
    • variables 12.5. env The env command without options will display a list of exported variables. The difference with set with options is that set lists all variables, including those not exported to child shells. But env can also be used to start a clean shell (a shell without any inherited environment). The env -i command clears the environment for the subshell. Notice in this screenshot that bash will set the $SHELL variable on startup. [paul@RHEL4b ~]$ bash -c 'echo $SHELL $HOME $USER' /bin/bash /home/paul paul [paul@RHEL4b ~]$ env -i bash -c 'echo $SHELL $HOME $USER' /bin/bash [paul@RHEL4b ~]$ You can use the env command to set the $LANG, or any other, variable for just one instance of bash with one command. The example below uses this to show the influence of the $LANG variable on file globbing (see the chapter on file globbing). [paul@RHEL4b test]$ env LANG=C bash -c 'ls File[a-z]' Filea Fileb [paul@RHEL4b test]$ env LANG=en_US.UTF-8 bash -c 'ls File[a-z]' Filea FileA Fileb FileB [paul@RHEL4b test]$ 12.6. export You can export shell variables to other shells with the export command. This will export the variable to child shells. [paul@RHEL4b [paul@RHEL4b [paul@RHEL4b [paul@RHEL4b three four [paul@RHEL4b [paul@RHEL4b four ~]$ ~]$ ~]$ ~]$ var3=three var4=four export var4 echo $var3 $var4 ~]$ bash ~]$ echo $var3 $var4 But it will not export to the parent shell (previous screenshot continued). [paul@RHEL4b [paul@RHEL4b four five [paul@RHEL4b exit [paul@RHEL4b three four [paul@RHEL4b ~]$ export var5=five ~]$ echo $var3 $var4 $var5 ~]$ exit ~]$ echo $var3 $var4 $var5 ~]$ 93
    • variables 12.7. delineate variables Until now, we have seen that bash interprets a variable starting from a dollar sign, continuing until the first occurrence of a non-alphanumeric character that is not an underscore. In some situations, this can be a problem. This issue can be resolved with curly braces like in this example. [paul@RHEL4b ~]$ prefix=Super [paul@RHEL4b ~]$ echo Hello $prefixman and $prefixgirl Hello and [paul@RHEL4b ~]$ echo Hello ${prefix}man and ${prefix}girl Hello Superman and Supergirl [paul@RHEL4b ~]$ 12.8. unbound variables The example below tries to display the value of the $MyVar variable, but it fails because the variable does not exist. By default the shell will display nothing when a variable is unbound (does not exist). [paul@RHELv4u3 gen]$ echo $MyVar [paul@RHELv4u3 gen]$ There is, however, the nounset shell option that you can use to generate an error when a variable does not exist. paul@laika:~$ set -u paul@laika:~$ echo $Myvar bash: Myvar: unbound variable paul@laika:~$ set +u paul@laika:~$ echo $Myvar paul@laika:~$ In the bash shell set -u is identical to set -o nounset and likewise set +u is identical to set +o nounset. 94
    • variables 12.9. shell options Both set and unset are builtin shell commands. They can be used to set options of the bash shell itself. The next example will clarify this. By default, the shell will treat unset variables as a variable having no value. By setting the -u option, the shell will treat any reference to unset variables as an error. See the man page of bash for more information. [paul@RHEL4b ~]$ echo $var123 [paul@RHEL4b ~]$ set -u [paul@RHEL4b ~]$ echo $var123 -bash: var123: unbound variable [paul@RHEL4b ~]$ set +u [paul@RHEL4b ~]$ echo $var123 [paul@RHEL4b ~]$ To list all the set options for your shell, use echo $-. The noclobber (or -C) option will be explained later in this book (in the I/O redirection chapter). [paul@RHEL4b himBH [paul@RHEL4b [paul@RHEL4b himuBCH [paul@RHEL4b [paul@RHEL4b himBH [paul@RHEL4b ~]$ echo $~]$ set -C ; set -u ~]$ echo $~]$ set +C ; set +u ~]$ echo $~]$ When typing set without options, you get a list of all variables without function when the shell is on posix mode. You can set bash in posix mode typing set -o posix. 95
    • variables 12.10. shell embedding Shells can be embedded on the command line, or in other words, the command line scan can spawn new processes containing a fork of the current shell. You can use variables to prove that new shells are created. In the screenshot below, the variable $var1 only exists in the (temporary) sub shell. [paul@RHELv4u3 gen]$ echo $var1 [paul@RHELv4u3 gen]$ echo $(var1=5;echo $var1) 5 [paul@RHELv4u3 gen]$ echo $var1 [paul@RHELv4u3 gen]$ You can embed a shell in an embedded shell, this is called nested embedding of shells. This screenshot shows an embedded shell inside an embedded shell. paul@deb503:~$ A=shell paul@deb503:~$ echo $C$B$A $(B=sub;echo $C$B$A; echo $(C=sub;echo $C$B$A)) shell subshell subsubshell backticks Single embedding can be useful to avoid changing your current directory. The screenshot below uses backticks instead of dollar-bracket to embed. [paul@RHELv4u3 ~]$ echo `cd /etc; ls -d * | grep pass` passwd passwd- passwd.OLD [paul@RHELv4u3 ~]$ You can only use the $() notation to nest embedded shells, backticks cannot do this. backticks or single quotes Placing the embedding between backticks uses one character less than the dollar and parenthesis combo. Be careful however, backticks are often confused with single quotes. The technical difference between ' and ` is significant! [paul@RHELv4u3 gen]$ echo `var1=5;echo $var1` 5 [paul@RHELv4u3 gen]$ echo 'var1=5;echo $var1' var1=5;echo $var1 [paul@RHELv4u3 gen]$ 96
    • variables 12.11. practice: shell variables 1. Use echo to display Hello followed by your username. (use a bash variable!) 2. Create a variable answer with a value of 42. 3. Copy the value of $LANG to $MyLANG. 4. List all current shell variables. 5. List all exported shell variables. 6. Do the env and set commands display your variable ? 6. Destroy your answer variable. 7. Find the list of shell options in the man page of bash. What is the difference between set -u and set -o nounset? 8. Create two variables, and export one of them. 9. Display the exported variable in an interactive child shell. 10. Create a variable, give it the value 'Dumb', create another variable with value 'do'. Use echo and the two variables to echo Dumbledore. 11. Activate nounset in your shell. Test that it shows an error message when using non-existing variables. 12. Deactivate nounset. 13. Find the list of backslash escaped characters in the manual of bash. Add the time to your PS1 prompt. 14. Execute cd /var and ls in an embedded shell. 15. Create the variable embvar in an embedded shell and echo it. Does the variable exist in your current shell now ? 16. Explain what "set -x" does. Can this be useful ? (optional)17. Given the following screenshot, add exactly four characters to that command line so that the total output is FirstMiddleLast. [paul@RHEL4b ~]$ echo First; echo Middle; echo Last 18. Display a long listing (ls -l) of the passwd command using the which command inside back ticks. 97
    • variables 12.12. solution: shell variables 1. Use echo to display Hello followed by your username. (use a bash variable!) echo Hello $USER 2. Create a variable answer with a value of 42. answer=42 3. Copy the value of $LANG to $MyLANG. MyLANG=$LANG 4. List all current shell variables. set set|more on Ubuntu/Debian 5. List all exported shell variables. env 6. Do the env and set commands display your variable ? env | more set | more 6. Destroy your answer variable. unset answer 7. Find the list of shell options in the man page of bash. What is the difference between set -u and set -o nounset? read the manual of bash (man bash), search for nounset -- both mean the same thing. 8. Create two variables, and export one of them. var1=1; export var2=2 9. Display the exported variable in an interactive child shell. bash echo $var2 10. Create a variable, give it the value 'Dumb', create another variable with value 'do'. Use echo and the two variables to echo Dumbledore. varx=Dumb; vary=do echo ${varx}le${vary}re solution by Yves from Dexia : echo $varx'le'$vary're' solution by Erwin from Telenet : echo "$varx"le"$vary"re 11. Activate nounset in your shell. Test that it shows an error message when using non-existing variables. 98
    • variables set -u OR set -o nounset Both these lines have the same effect. 12. Deactivate nounset. set +u OR set +o nounset 13. Find the list of backslash escaped characters in the manual of bash. Add the time to your PS1 prompt. PS1='t u@h W$ ' 14. Execute cd /var and ls in an embedded shell. echo $(cd /var ; ls) The echo command is only needed to show the result of the ls command. Omitting will result in the shell trying to execute the first file as a command. 15. Create the variable embvar in an embedded shell and echo it. Does the variable exist in your current shell now ? $(embvar=emb;echo $embvar) ; echo $embvar (the last echo fails). $embvar does not exist in your current shell 16. Explain what "set -x" does. Can this be useful ? It displays shell expansion for troubleshooting your command. (optional)17. Given the following screenshot, add exactly four characters to that command line so that the total output is FirstMiddleLast. [paul@RHEL4b ~]$ echo First; echo Middle; echo Last echo -n First; echo -n Middle; echo Last 18. Display a long listing (ls -l) of the passwd command using the which command inside back ticks. ls -l `which passwd` 99
    • Chapter 13. shell history Table of Contents 13.1. repeating the last command ....................................................................... 101 13.2. repeating other commands ......................................................................... 101 13.3. history ......................................................................................................... 101 13.4. !n ................................................................................................................. 101 13.5. Ctrl-r ........................................................................................................... 102 13.6. $HISTSIZE ................................................................................................ 102 13.7. $HISTFILE ................................................................................................ 102 13.8. $HISTFILESIZE ........................................................................................ 102 13.9. (optional)regular expressions ..................................................................... 103 13.10. (optional)repeating commands in ksh ...................................................... 103 13.11. practice: shell history ............................................................................... 104 13.12. solution: shell history ............................................................................... 105 The shell makes it easy for us to repeat commands, this chapter explains how. 100
    • shell history 13.1. repeating the last command To repeat the last command in bash, type !!. This is pronounced as bang bang. paul@debian5:~/test42$ echo this will be repeated > file42.txt paul@debian5:~/test42$ !! echo this will be repeated > file42.txt paul@debian5:~/test42$ 13.2. repeating other commands You can repeat other commands using one bang followed by one or more characters. The shell will repeat the last command that started with those characters. paul@debian5:~/test42$ touch file42 paul@debian5:~/test42$ cat file42 paul@debian5:~/test42$ !to touch file42 paul@debian5:~/test42$ 13.3. history To see older commands, use history to display the shell command history (or use history n to see the last n commands). paul@debian5:~/test$ history 10 38 mkdir test 39 cd test 40 touch file1 41 echo hello > file2 42 echo It is very cold today > winter.txt 43 ls 44 ls -l 45 cp winter.txt summer.txt 46 ls -l 47 history 10 13.4. !n When typing ! followed by the number preceding the command you want repeated, then the shell will echo the command and execute it. paul@debian5:~/test$ !43 ls file1 file2 summer.txt winter.txt 101
    • shell history 13.5. Ctrl-r Another option is to use ctrl-r to search in the history. In the screenshot below i only typed ctrl-r followed by four characters apti and it finds the last command containing these four consecutive characters. paul@debian5:~$ (reverse-i-search)`apti': sudo aptitude install screen 13.6. $HISTSIZE The $HISTSIZE variable determines the number of commands that will be remembered in your current environment. Most distributions default this variable to 500 or 1000. paul@debian5:~$ echo $HISTSIZE 500 You can change it to any value you like. paul@debian5:~$ HISTSIZE=15000 paul@debian5:~$ echo $HISTSIZE 15000 13.7. $HISTFILE The $HISTFILE variable points to the file that contains your history. The bash shell defaults this value to ~/.bash_history. paul@debian5:~$ echo $HISTFILE /home/paul/.bash_history A session history is saved to this file when you exit the session! Closing a gnome-terminal with the mouse, or typing reboot as root will NOT save your terminal's history. 13.8. $HISTFILESIZE The number of commands kept in your history file can be set using $HISTFILESIZE. paul@debian5:~$ echo $HISTFILESIZE 15000 102
    • shell history 13.9. (optional)regular expressions It is possible to use regular expressions when using the bang to repeat commands. The screenshot below switches 1 into 2. paul@deianb5:~/test$ cat file1 paul@debian5:~/test$ !c:s/1/2 cat file2 hello paul@debian5:~/test$ 13.10. (optional)repeating commands in ksh Repeating a command in the Korn shell is very similar. The Korn shell also has the history command, but uses the letter r to recall lines from history. This screenshot shows the history command. Note the different meaning of the parameter. $ history 17 17 clear 18 echo hoi 19 history 12 20 echo world 21 history 17 Repeating with r can be combined with the line numbers given by the history command, or with the first few letters of the command. $ r e echo world world $ cd /etc $ r cd /etc $ 103
    • shell history 13.11. practice: shell history 1. Issue the command echo The answer to the meaning of life, the universe and everything is 42. 2. Repeat the previous command using only two characters (there are two solutions!) 3. Display the last 5 commands you typed. 4. Issue the long echo from question 1 again, using the line numbers you received from the command in question 3. 5. How many commands can be kept in memory for your current shell session ? 6. Where are these commands stored when exiting the shell ? 7. How many commands can be written to the history file when exiting your current shell session ? 8. Make sure your current bash shell remembers the next 5000 commands you type. 9. Open more than one console (press Ctrl-shift-t in gnome-terminal) with the same user account. When is command history written to the history file ? 104
    • shell history 13.12. solution: shell history 1. Issue the command echo The answer to the meaning of life, the universe and everything is 42. echo The answer to the meaning of life, the universe and everything is 42 2. Repeat the previous command using only two characters (there are two solutions!) !! OR !e 3. Display the last 5 commands you typed. paul@ubu1010:~$ history 5 52 ls -l 53 ls 54 df -h | grep sda 55 echo The answer to the meaning of life, the universe and everything is 42 56 history 5 You will receive different line numbers. 4. Issue the long echo from question 1 again, using the line numbers you received from the command in question 3. paul@ubu1010:~$ !56 echo The answer to the meaning of life, the universe and everything is 42 The answer to the meaning of life, the universe and everything is 42 5. How many commands can be kept in memory for your current shell session ? echo $HISTSIZE 6. Where are these commands stored when exiting the shell ? echo $HISTFILE 7. How many commands can be written to the history file when exiting your current shell session ? echo $HISTFILESIZE 8. Make sure your current bash shell remembers the next 5000 commands you type. HISTSIZE=5000 9. Open more than one console (press Ctrl-shift-t in gnome-terminal) with the same user account. When is command history written to the history file ? when you type exit 105
    • Chapter 14. file globbing Table of Contents 14.1. 14.2. 14.3. 14.4. 14.5. 14.6. 14.7. 14.8. * asterisk .................................................................................................... ? question mark .......................................................................................... [] square brackets ....................................................................................... a-z and 0-9 ranges ..................................................................................... $LANG and square brackets ...................................................................... preventing file globbing ............................................................................. practice: shell globbing .............................................................................. solution: shell globbing .............................................................................. 107 107 107 108 108 109 110 111 The shell is also responsible for file globbing (or dynamic filename generation). This chapter will explain file globbing. 106
    • file globbing 14.1. * asterisk The asterisk * is interpreted by the shell as a sign to generate filenames, matching the asterisk to any combination of characters (even none). When no path is given, the shell will use filenames in the current directory. See the man page of glob(7) for more information. (This is part of LPI topic 1.103.3.) [paul@RHELv4u3 gen]$ file1 file2 file3 [paul@RHELv4u3 gen]$ File4 File55 FileA [paul@RHELv4u3 gen]$ file1 file2 file3 [paul@RHELv4u3 gen]$ File55 [paul@RHELv4u3 gen]$ File55 [paul@RHELv4u3 gen]$ File55 [paul@RHELv4u3 gen]$ ls File4 File55 FileA ls File* Fileab FileAB ls file* fileab fileabc ls *ile55 fileab Fileab FileAB fileabc ls F*ile55 ls F*55 14.2. ? question mark Similar to the asterisk, the question mark ? is interpreted by the shell as a sign to generate filenames, matching the question mark with exactly one character. [paul@RHELv4u3 gen]$ ls file1 file2 file3 File4 File55 [paul@RHELv4u3 gen]$ ls File? File4 FileA [paul@RHELv4u3 gen]$ ls Fil?4 File4 [paul@RHELv4u3 gen]$ ls Fil?? File4 FileA [paul@RHELv4u3 gen]$ ls File?? File55 Fileab FileAB [paul@RHELv4u3 gen]$ FileA fileab Fileab FileAB fileabc 14.3. [] square brackets The square bracket [ is interpreted by the shell as a sign to generate filenames, matching any of the characters between [ and the first subsequent ]. The order in this list between the brackets is not important. Each pair of brackets is replaced by exactly one character. [paul@RHELv4u3 gen]$ file1 file2 file3 [paul@RHELv4u3 gen]$ FileA [paul@RHELv4u3 gen]$ FileA [paul@RHELv4u3 gen]$ File55 [paul@RHELv4u3 gen]$ File55 Fileab ls File4 File55 ls File[5A] ls File[A5] ls File[A5][5b] ls File[a5][5b] 107 FileA fileab Fileab FileAB fileabc
    • file globbing [paul@RHELv4u3 gen]$ ls File[a5][5b][abcdefghijklm] ls: File[a5][5b][abcdefghijklm]: No such file or directory [paul@RHELv4u3 gen]$ ls file[a5][5b][abcdefghijklm] fileabc [paul@RHELv4u3 gen]$ You can also exclude characters from a list between square brackets with the exclamation mark !. And you are allowed to make combinations of these wild cards. [paul@RHELv4u3 gen]$ file1 file2 file3 [paul@RHELv4u3 gen]$ fileab [paul@RHELv4u3 gen]$ file1 file2 file3 [paul@RHELv4u3 gen]$ fileab [paul@RHELv4u3 gen]$ ls File4 File55 FileA ls file[a5][!Z] fileab Fileab FileAB fileabc ls file[!5]* fileab fileabc ls file[!5]? 14.4. a-z and 0-9 ranges The bash shell will also understand ranges of characters between brackets. [paul@RHELv4u3 gen]$ ls file1 file3 File55 fileab FileAB fileabc file2 File4 FileA Fileab fileab2 [paul@RHELv4u3 gen]$ ls file[a-z]* fileab fileab2 fileabc [paul@RHELv4u3 gen]$ ls file[0-9] file1 file2 file3 [paul@RHELv4u3 gen]$ ls file[a-z][a-z][0-9]* fileab2 [paul@RHELv4u3 gen]$ 14.5. $LANG and square brackets But, don't forget the influence of the LANG variable. Some languages include lower case letters in an upper case range (and vice versa). paul@RHELv4u4:~/test$ ls [A-Z]ile? file1 file2 file3 File4 paul@RHELv4u4:~/test$ ls [a-z]ile? file1 file2 file3 File4 paul@RHELv4u4:~/test$ echo $LANG en_US.UTF-8 paul@RHELv4u4:~/test$ LANG=C paul@RHELv4u4:~/test$ echo $LANG C paul@RHELv4u4:~/test$ ls [a-z]ile? file1 file2 file3 paul@RHELv4u4:~/test$ ls [A-Z]ile? File4 paul@RHELv4u4:~/test$ 108
    • file globbing 14.6. preventing file globbing The screenshot below should be no surprise. The echo * will echo a * when in an empty directory. And it will echo the names of all files when the directory is not empty. paul@ubu1010:~$ mkdir test42 paul@ubu1010:~$ cd test42 paul@ubu1010:~/test42$ echo * * paul@ubu1010:~/test42$ touch file42 file33 paul@ubu1010:~/test42$ echo * file33 file42 Globbing can be prevented using quotes or by escaping the special characters, as shown in this screenshot. paul@ubu1010:~/test42$ file33 file42 paul@ubu1010:~/test42$ * paul@ubu1010:~/test42$ * paul@ubu1010:~/test42$ * echo * echo * echo '*' echo "*" 109
    • file globbing 14.7. practice: shell globbing 1. Create a test directory and enter it. 2. Create files file1 file10 file11 file2 File2 File3 file33 fileAB filea fileA fileAAA file( file 2 (the last one has 6 characters including a space) 3. List (with ls) all files starting with file 4. List (with ls) all files starting with File 5. List (with ls) all files starting with file and ending in a number. 6. List (with ls) all files starting with file and ending with a letter 7. List (with ls) all files starting with File and having a digit as fifth character. 8. List (with ls) all files starting with File and having a digit as fifth character and nothing else. 9. List (with ls) all files starting with a letter and ending in a number. 10. List (with ls) all files that have exactly five characters. 11. List (with ls) all files that start with f or F and end with 3 or A. 12. List (with ls) all files that start with f have i or R as second character and end in a number. 13. List all files that do not start with the letter F. 14. Copy the value of $LANG to $MyLANG. 15. Show the influence of $LANG in listing A-Z or a-z ranges. 16. You receive information that one of your servers was cracked, the cracker probably replaced the ls command. You know that the echo command is safe to use. Can echo replace ls ? How can you list the files in the current directory with echo ? 17. Is there another command besides cd to change directories ? 110
    • file globbing 14.8. solution: shell globbing 1. Create a test directory and enter it. mkdir testdir; cd testdir 2. Create files file1 file10 file11 file2 File2 File3 file33 fileAB filea fileA fileAAA file( file 2 (the last one has 6 characters including a space) touch touch touch touch file1 file10 file11 file2 File2 File3 file33 fileAB filea fileA fileAAA "file(" "file 2" 3. List (with ls) all files starting with file ls file* 4. List (with ls) all files starting with File ls File* 5. List (with ls) all files starting with file and ending in a number. ls file*[0-9] 6. List (with ls) all files starting with file and ending with a letter ls file*[a-z] 7. List (with ls) all files starting with File and having a digit as fifth character. ls File[0-9]* 8. List (with ls) all files starting with File and having a digit as fifth character and nothing else. ls File[0-9] 9. List (with ls) all files starting with a letter and ending in a number. ls [a-z]*[0-9] 10. List (with ls) all files that have exactly five characters. ls ????? 11. List (with ls) all files that start with f or F and end with 3 or A. ls [fF]*[3A] 12. List (with ls) all files that start with f have i or R as second character and end in a number. ls f[iR]*[0-9] 13. List all files that do not start with the letter F. ls [!F]* 111
    • file globbing 14. Copy the value of $LANG to $MyLANG. MyLANG=$LANG 15. Show the influence of $LANG in listing A-Z or a-z ranges. see example in book 16. You receive information that one of your servers was cracked, the cracker probably replaced the ls command. You know that the echo command is safe to use. Can echo replace ls ? How can you list the files in the current directory with echo ? echo * 17. Is there another command besides cd to change directories ? pushd popd 112
    • Part IV. pipes and commands
    • Chapter 15. redirection and pipes Table of Contents 15.1. stdin, stdout, and stderr .............................................................................. 15.2. output redirection ....................................................................................... 15.3. error redirection ......................................................................................... 15.4. input redirection ......................................................................................... 15.5. confusing redirection .................................................................................. 15.6. quick file clear ........................................................................................... 15.7. swapping stdout and stderr ........................................................................ 15.8. pipes ........................................................................................................... 15.9. practice: redirection and pipes ................................................................... 15.10. solution: redirection and pipes ................................................................. 115 115 117 118 119 119 119 120 121 122 One of the powers of the Unix command line is the use of redirection and pipes. This chapter first explains redirection of input, output and error streams. It then introduces pipes that consist of several commands. 114
    • redirection and pipes 15.1. stdin, stdout, and stderr The shell (and almost every other Linux command) takes input from stdin (stream 0) and sends output to stdout (stream 1) and error messages to stderr (stream 2) . The keyboard often serves as stdin, stdout and stderr both go to the display. The shell allows you to redirect these streams. 15.2. output redirection > stdout stdout can be redirected with a greater than sign. While scanning the line, the shell will see the > sign and will clear the file. [paul@RHELv4u3 ~]$ echo It is cold today! It is cold today! [paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! [paul@RHELv4u3 ~]$ Note that the > notation is in fact the abbreviation of 1> (stdout being referred to as stream 1. output file is erased To repeat: While scanning the line, the shell will see the > sign and will clear the file! This means that even when the command fails, the file will be cleared! [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! [paul@RHELv4u3 ~]$ zcho It is cold today! > winter.txt -bash: zcho: command not found [paul@RHELv4u3 ~]$ cat winter.txt [paul@RHELv4u3 ~]$ noclobber Erasing a file while using > can be prevented by setting the noclobber option. [paul@RHELv4u3 ~]$ It is cold today! [paul@RHELv4u3 ~]$ [paul@RHELv4u3 ~]$ -bash: winter.txt: [paul@RHELv4u3 ~]$ [paul@RHELv4u3 ~]$ cat winter.txt set -o noclobber echo It is cold today! > winter.txt cannot overwrite existing file set +o noclobber 115
    • redirection and pipes overruling noclobber The noclobber can be overruled with >|. [paul@RHELv4u3 ~]$ set -o noclobber [paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt -bash: winter.txt: cannot overwrite existing file [paul@RHELv4u3 ~]$ echo It is very cold today! >| winter.txt [paul@RHELv4u3 ~]$ cat winter.txt It is very cold today! [paul@RHELv4u3 ~]$ >> append Use >> to append output to a file. [paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! [paul@RHELv4u3 ~]$ echo Where is the summer ? >> winter.txt [paul@RHELv4u3 ~]$ cat winter.txt It is cold today! Where is the summer ? [paul@RHELv4u3 ~]$ 116
    • redirection and pipes 15.3. error redirection 2> stderr Redirecting stderr is done with 2>. This can be very useful to prevent error messages from cluttering your screen. The screenshot below shows redirection of stdout to a file, and stderr to /dev/null. Writing 1> is the same as >. [paul@RHELv4u3 ~]$ find / > allfiles.txt 2> /dev/null [paul@RHELv4u3 ~]$ 2>&1 To redirect both stdout and stderr to the same file, use 2>&1. [paul@RHELv4u3 ~]$ find / > allfiles_and_errors.txt 2>&1 [paul@RHELv4u3 ~]$ Note that the order of redirections is significant. For example, the command ls > dirlist 2>&1 directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command ls 2>&1 > dirlist directs only the standard output to file dirlist, because the standard error made a copy of the standard output before the standard output was redirected to dirlist. 117
    • redirection and pipes 15.4. input redirection < stdin Redirecting stdin is done with < (short for 0<). [paul@RHEL4b ~]$ cat < text.txt one two [paul@RHEL4b ~]$ tr 'onetw' 'ONEZZ' < text.txt ONE ZZO [paul@RHEL4b ~]$ << here document The here document (sometimes called here-is-document) is a way to append input until a certain sequence (usually EOF) is encountered. The EOF marker can be typed literally or can be called with Ctrl-D. [paul@RHEL4b > one > two > EOF [paul@RHEL4b one two [paul@RHEL4b > brel > brol [paul@RHEL4b brel [paul@RHEL4b ~]$ cat <<EOF > text.txt ~]$ cat text.txt ~]$ cat <<brol > text.txt ~]$ cat text.txt ~]$ <<< here string The here string can be used to directly pass strings to a command. The result is the same as using echo string | command (but you have one less process running). paul@ubu1110~$ base64 <<< linux-training.be bGludXgtdHJhaW5pbmcuYmUK paul@ubu1110~$ base64 -d <<< bGludXgtdHJhaW5pbmcuYmUK linux-training.be See rfc 3548 for more information about base64. 118
    • redirection and pipes 15.5. confusing redirection The shell will scan the whole line before applying redirection. The following command line is very readable and is correct. cat winter.txt > snow.txt 2> errors.txt But this one is also correct, but less readable. 2> errors.txt cat winter.txt > snow.txt Even this will be understood perfectly by the shell. < winter.txt > snow.txt 2> errors.txt cat 15.6. quick file clear So what is the quickest way to clear a file ? >foo And what is the quickest way to clear a file when the noclobber option is set ? >|bar 15.7. swapping stdout and stderr When filtering an output stream, e.g. through a regular pipe ( | ) you only can filter stdout. Say you want to filter out some unimportant error, out of the stderr stream. This cannot be done directly, and you need to 'swap' stdout and stderr. This can be done by using a 4th stream referred to with number 3: 3>&1 1>&2 2>&3 This Tower Of Hanoi like construction uses a temporary stream 3, to be able to swap stdout (1) and stderr (2). The following is an example of how to filter out all lines in the stderr stream, containing $error. $command 3>&1 1>&2 2>&3 | grep -v $error 3>&1 1>&2 2>&3 But in this example, it can be done in a much shorter way, by using a pipe on STDERR: /usr/bin/$somecommand |& grep -v $error 119
    • redirection and pipes 15.8. pipes One of the most powerful advantages of Linux is the use of pipes. A pipe takes stdout from the previous command and sends it as stdin to the next command. All commands in a pipe run simultaneously. | vertical bar Consider the following example. paul@debian5:~/test$ ls /etc > etcfiles.txt paul@debian5:~/test$ tail -4 etcfiles.txt X11 xdg xml xpdf paul@debian5:~/test$ This can be written in one command line using a pipe. paul@debian5:~/test$ ls /etc | tail -4 X11 xdg xml xpdf paul@debian5:~/test$ The pipe is represented by a vertical bar | between two commands. multiple pipes One command line can use multiple pipes. All commands in the pipe can run at the same time. paul@deb503:~/test$ ls /etc | tail -4 | tac xpdf xml xdg X11 120
    • redirection and pipes 15.9. practice: redirection and pipes 1. Use ls to output the contents of the /etc/ directory to a file called etc.txt. 2. Activate the noclobber shell option. 3. Verify that nocclobber is active by repeating your ls on /etc/. 4. When listing all shell options, which character represents the noclobber option ? 5. Deactivate the noclobber option. 6. Make sure you have two shells open on the same computer. Create an empty tailing.txt file. Then type tail -f tailing.txt. Use the second shell to append a line of text to that file. Verify that the first shell displays this line. 7. Create a file that contains the names of five people. Use cat and output redirection to create the file and use a here document to end the input. 121
    • redirection and pipes 15.10. solution: redirection and pipes 1. Use ls to output the contents of the /etc/ directory to a file called etc.txt. ls /etc > etc.txt 2. Activate the noclobber shell option. set -o noclobber 3. Verify that nocclobber is active by repeating your ls on /etc/. ls /etc > etc.txt (should not work) 4. When listing all shell options, which character represents the noclobber option ? echo $- (noclobber is visible as C) 5. Deactivate the noclobber option. set +o noclobber 6. Make sure you have two shells open on the same computer. Create an empty tailing.txt file. Then type tail -f tailing.txt. Use the second shell to append a line of text to that file. Verify that the first shell displays this line. paul@deb503:~$ > tailing.txt paul@deb503:~$ tail -f tailing.txt hello world in the other shell: paul@deb503:~$ echo hello >> tailing.txt paul@deb503:~$ echo world >> tailing.txt 7. Create a file that contains the names of five people. Use cat and output redirection to create the file and use a here document to end the input. paul@deb503:~$ cat > tennis.txt << ace > Justine Henin > Venus Williams > Serena Williams > Martina Hingis > Kim Clijsters > ace paul@deb503:~$ cat tennis.txt Justine Henin Venus Williams Serena Williams Martina Hingis Kim Clijsters paul@deb503:~$ 122
    • Chapter 16. filters Table of Contents 16.1. cat ............................................................................................................... 16.2. tee ............................................................................................................... 16.3. grep ............................................................................................................. 16.4. cut ............................................................................................................... 16.5. tr ................................................................................................................. 16.6. wc ............................................................................................................... 16.7. sort .............................................................................................................. 16.8. uniq ............................................................................................................. 16.9. comm .......................................................................................................... 16.10. od .............................................................................................................. 16.11. sed ............................................................................................................ 16.12. pipe examples ........................................................................................... 16.13. practice: filters ......................................................................................... 16.14. solution: filters ......................................................................................... 124 124 124 126 126 127 128 129 129 130 131 132 133 134 Commands that are created to be used with a pipe are often called filters. These filters are very small programs that do one specific thing very efficiently. They can be used as building blocks. This chapter will introduce you to the most common filters. The combination of simple commands and filters in a long pipe allows you to design elegant solutions. 123
    • filters 16.1. cat When between two pipes, the cat command does nothing (except putting stdin on stdout. [paul@RHEL4b pipes]$ tac count.txt | cat | cat | cat | cat | cat five four three two one [paul@RHEL4b pipes]$ 16.2. tee Writing long pipes in Unix is fun, but sometimes you might want intermediate results. This is were tee comes in handy. The tee filter puts stdin on stdout and also into a file. So tee is almost the same as cat, except that it has two identical outputs. [paul@RHEL4b pipes]$ tac count.txt | tee temp.txt | tac one two three four five [paul@RHEL4b pipes]$ cat temp.txt five four three two one [paul@RHEL4b pipes]$ 16.3. grep The grep filter is famous among Unix users. The most common use of grep is to filter lines of text containing (or not containing) a certain string. [paul@RHEL4b pipes]$ cat tennis.txt Amelie Mauresmo, Fra Kim Clijsters, BEL Justine Henin, Bel Serena Williams, usa Venus Williams, USA [paul@RHEL4b pipes]$ cat tennis.txt | grep Williams Serena Williams, usa Venus Williams, USA You can write this without the cat. [paul@RHEL4b pipes]$ grep Williams tennis.txt Serena Williams, usa Venus Williams, USA One of the most useful options of grep is grep -i which filters in a case insensitive way. 124
    • filters [paul@RHEL4b pipes]$ grep Bel tennis.txt Justine Henin, Bel [paul@RHEL4b pipes]$ grep -i Bel tennis.txt Kim Clijsters, BEL Justine Henin, Bel [paul@RHEL4b pipes]$ Another very useful option is grep -v which outputs lines not matching the string. [paul@RHEL4b pipes]$ grep -v Fra tennis.txt Kim Clijsters, BEL Justine Henin, Bel Serena Williams, usa Venus Williams, USA [paul@RHEL4b pipes]$ And of course, both options can be combined to filter all lines not containing a case insensitive string. [paul@RHEL4b pipes]$ grep -vi usa tennis.txt Amelie Mauresmo, Fra Kim Clijsters, BEL Justine Henin, Bel [paul@RHEL4b pipes]$ With grep -A1 one line after the result is also displayed. paul@debian5:~/pipes$ grep -A1 Henin tennis.txt Justine Henin, Bel Serena Williams, usa With grep -B1 one line before the result is also displayed. paul@debian5:~/pipes$ grep -B1 Henin tennis.txt Kim Clijsters, BEL Justine Henin, Bel With grep -C1 (context) one line before and one after are also displayed. All three options (A,B, and C) can display any number of lines (using e.g. A2, B4 or C20). paul@debian5:~/pipes$ grep -C1 Henin tennis.txt Kim Clijsters, BEL Justine Henin, Bel Serena Williams, usa 125
    • filters 16.4. cut The cut filter can select columns from files, depending on a delimiter or a count of bytes. The screenshot below uses cut to filter for the username and userid in the /etc/ passwd file. It uses the colon as a delimiter, and selects fields 1 and 3. [[paul@RHEL4b pipes]$ cut -d: -f1,3 /etc/passwd | tail -4 Figo:510 Pfaff:511 Harry:516 Hermione:517 [paul@RHEL4b pipes]$ When using a space as the delimiter for cut, you have to quote the space. [paul@RHEL4b pipes]$ cut -d" " -f1 tennis.txt Amelie Kim Justine Serena Venus [paul@RHEL4b pipes]$ This example uses cut to display the second to the seventh character of /etc/passwd. [paul@RHEL4b pipes]$ cut -c2-7 /etc/passwd | tail -4 igo:x: faff:x arry:x ermion [paul@RHEL4b pipes]$ 16.5. tr You can translate characters with tr. The screenshot shows the translation of all occurrences of e to E. [paul@RHEL4b pipes]$ cat tennis.txt | tr 'e' 'E' AmEliE MaurEsmo, Fra Kim ClijstErs, BEL JustinE HEnin, BEl SErEna Williams, usa VEnus Williams, USA Here we set all letters to uppercase by defining two ranges. [paul@RHEL4b pipes]$ cat tennis.txt | tr 'a-z' 'A-Z' AMELIE MAURESMO, FRA KIM CLIJSTERS, BEL JUSTINE HENIN, BEL SERENA WILLIAMS, USA VENUS WILLIAMS, USA [paul@RHEL4b pipes]$ Here we translate all newlines to spaces. [paul@RHEL4b pipes]$ cat count.txt one 126
    • filters two three four five [paul@RHEL4b pipes]$ cat count.txt | tr 'n' ' ' one two three four five [paul@RHEL4b pipes]$ The tr -s filter can also be used to squeeze multiple occurrences of a character to one. [paul@RHEL4b pipes]$ cat spaces.txt one two three four five six [paul@RHEL4b pipes]$ cat spaces.txt | tr -s ' ' one two three four five six [paul@RHEL4b pipes]$ You can also use tr to 'encrypt' texts with rot13. [paul@RHEL4b pipes]$ cat count.txt | tr 'a-z' 'nopqrstuvwxyzabcdefghijklm' bar gjb guerr sbhe svir [paul@RHEL4b pipes]$ cat count.txt | tr 'a-z' 'n-za-m' bar gjb guerr sbhe svir [paul@RHEL4b pipes]$ This last example uses tr -d to delete characters. paul@debian5:~/pipes$ cat tennis.txt | tr -d e Amli Maursmo, Fra Kim Clijstrs, BEL Justin Hnin, Bl Srna Williams, usa Vnus Williams, USA 16.6. wc Counting words, lines and characters is easy with wc. [paul@RHEL4b pipes]$ wc 5 15 100 tennis.txt [paul@RHEL4b pipes]$ wc 5 tennis.txt [paul@RHEL4b pipes]$ wc 15 tennis.txt [paul@RHEL4b pipes]$ wc 100 tennis.txt [paul@RHEL4b pipes]$ tennis.txt -l tennis.txt -w tennis.txt -c tennis.txt 127
    • filters 16.7. sort The sort filter will default to an alphabetical sort. paul@debian5:~/pipes$ cat music.txt Queen Brel Led Zeppelin Abba paul@debian5:~/pipes$ sort music.txt Abba Brel Led Zeppelin Queen But the sort filter has many options to tweak its usage. This example shows sorting different columns (column 1 or column 2). [paul@RHEL4b pipes]$ sort -k1 country.txt Belgium, Brussels, 10 France, Paris, 60 Germany, Berlin, 100 Iran, Teheran, 70 Italy, Rome, 50 [paul@RHEL4b pipes]$ sort -k2 country.txt Germany, Berlin, 100 Belgium, Brussels, 10 France, Paris, 60 Italy, Rome, 50 Iran, Teheran, 70 The screenshot below shows the difference between an alphabetical sort and a numerical sort (both on the third column). [paul@RHEL4b pipes]$ sort -k3 country.txt Belgium, Brussels, 10 Germany, Berlin, 100 Italy, Rome, 50 France, Paris, 60 Iran, Teheran, 70 [paul@RHEL4b pipes]$ sort -n -k3 country.txt Belgium, Brussels, 10 Italy, Rome, 50 France, Paris, 60 Iran, Teheran, 70 Germany, Berlin, 100 128
    • filters 16.8. uniq With uniq you can remove duplicates from a sorted list. paul@debian5:~/pipes$ cat music.txt Queen Brel Queen Abba paul@debian5:~/pipes$ sort music.txt Abba Brel Queen Queen paul@debian5:~/pipes$ sort music.txt |uniq Abba Brel Queen uniq can also count occurrences with the -c option. paul@debian5:~/pipes$ sort music.txt |uniq -c 1 Abba 1 Brel 2 Queen 16.9. comm Comparing streams (or files) can be done with the comm. By default comm will output three columns. In this example, Abba, Cure and Queen are in both lists, Bowie and Sweet are only in the first file, Turner is only in the second. paul@debian5:~/pipes$ cat > list1.txt Abba Bowie Cure Queen Sweet paul@debian5:~/pipes$ cat > list2.txt Abba Cure Queen Turner paul@debian5:~/pipes$ comm list1.txt list2.txt Abba Bowie Cure Queen Sweet Turner 129
    • filters The output of comm can be easier to read when outputting only a single column. The digits point out which output columns should not be displayed. paul@debian5:~/pipes$ comm -12 list1.txt list2.txt Abba Cure Queen paul@debian5:~/pipes$ comm -13 list1.txt list2.txt Turner paul@debian5:~/pipes$ comm -23 list1.txt list2.txt Bowie Sweet 16.10. od European humans like to work with ascii characters, but computers store files in bytes. The example below creates a simple file, and then uses od to show the contents of the file in hexadecimal bytes paul@laika:~/test$ cat > text.txt abcdefg 1234567 paul@laika:~/test$ od -t x1 text.txt 0000000 61 62 63 64 65 66 67 0a 31 32 33 34 35 36 37 0a 0000020 The same file can also be displayed in octal bytes. paul@laika:~/test$ od -b text.txt 0000000 141 142 143 144 145 146 147 012 061 062 063 064 065 066 067 012 0000020 And here is the file in ascii (or backslashed) characters. paul@laika:~/test$ od -c text.txt 0000000 a b c d e f g 0000020 130 n 1 2 3 4 5 6 7 n
    • filters 16.11. sed The stream editor sed can perform editing functions in the stream, using regular expressions. paul@debian5:~/pipes$ echo level5 | sed 's/5/42/' level42 paul@debian5:~/pipes$ echo level5 | sed 's/level/jump/' jump5 Add g for global replacements (all occurrences of the string per line). paul@debian5:~/pipes$ echo level5 level7 | sed 's/level/jump/' jump5 level7 paul@debian5:~/pipes$ echo level5 level7 | sed 's/level/jump/g' jump5 jump7 With d you can remove lines from a stream containing a character. paul@debian5:~/test42$ cat tennis.txt Venus Williams, USA Martina Hingis, SUI Justine Henin, BE Serena williams, USA Kim Clijsters, BE Yanina Wickmayer, BE paul@debian5:~/test42$ cat tennis.txt | sed '/BE/d' Venus Williams, USA Martina Hingis, SUI Serena williams, USA 131
    • filters 16.12. pipe examples who | wc How many users are logged on to this system ? [paul@RHEL4b pipes]$ who root tty1 Jul 25 10:50 paul pts/0 Jul 25 09:29 (laika) Harry pts/1 Jul 25 12:26 (barry) paul pts/2 Jul 25 12:26 (pasha) [paul@RHEL4b pipes]$ who | wc -l 4 who | cut | sort Display a sorted list of logged on users. [paul@RHEL4b pipes]$ who | cut -d' ' -f1 | sort Harry paul paul root Display a sorted list of logged on users, but every user only once . [paul@RHEL4b pipes]$ who | cut -d' ' -f1 | sort | uniq Harry paul root grep | cut Display a list of all bash user accounts on this computer. Users accounts are explained in detail later. paul@debian5:~$ grep bash /etc/passwd root:x:0:0:root:/root:/bin/bash paul:x:1000:1000:paul,,,:/home/paul:/bin/bash serena:x:1001:1001::/home/serena:/bin/bash paul@debian5:~$ grep bash /etc/passwd | cut -d: -f1 root paul serena 132
    • filters 16.13. practice: filters 1. Put a sorted list of all bash users in bashusers.txt. 2. Put a sorted list of all logged on users in onlineusers.txt. 3. Make a list of all filenames in /etc that contain the string samba. 4. Make a sorted list of all files in /etc that contain the case insensitive string samba. 5. Look at the output of /sbin/ifconfig. Write a line that displays only ip address and the subnet mask. 6. Write a line that removes all non-letters from a stream. 7. Write a line that receives a text file, and outputs all words on a separate line. 8. Write a spell checker on the command line. (There might be a dictionary in /usr/ share/dict/ .) 133
    • filters 16.14. solution: filters 1. Put a sorted list of all bash users in bashusers.txt. grep bash /etc/passwd | cut -d: -f1 | sort > bashusers.txt 2. Put a sorted list of all logged on users in onlineusers.txt. who | cut -d' ' -f1 | sort > onlineusers.txt 3. Make a list of all filenames in /etc that contain the string samba. ls /etc | grep samba 4. Make a sorted list of all files in /etc that contain the case insensitive string samba. ls /etc | grep -i samba | sort 5. Look at the output of /sbin/ifconfig. Write a line that displays only ip address and the subnet mask. /sbin/ifconfig | head -2 | grep 'inet ' | tr -s ' ' | cut -d' ' -f3,5 6. Write a line that removes all non-letters from a stream. paul@deb503:~$ cat text This is, yes really! , a text with ?&* too many str$ange# characters ;-) paul@deb503:~$ cat text | tr -d ',!$?.*&^%#@;()-' This is yes really a text with too many strange characters 7. Write a line that receives a text file, and outputs all words on a separate line. paul@deb503:~$ cat text2 it is very cold today without the sun paul@deb503:~$ cat text2 | tr ' ' 'n' it is very cold today without the sun 8. Write a spell checker on the command line. (There might be a dictionary in /usr/ share/dict/ .) paul@rhel ~$ echo "The zun is shining today" > text paul@rhel ~$ cat > DICT is shining sun the today 134
    • filters paul@rhel ~$ cat text | tr 'A-Z ' 'a-zn' | sort | uniq | comm -23 - DICT zun You could also add the solution from question number 6 to remove non-letters, and tr -s ' ' to remove redundant spaces. 135
    • Chapter 17. basic Unix tools Table of Contents 17.1. find ............................................................................................................. 17.2. locate .......................................................................................................... 17.3. date ............................................................................................................. 17.4. cal ............................................................................................................... 17.5. sleep ........................................................................................................... 17.6. time ............................................................................................................. 17.7. gzip - gunzip .............................................................................................. 17.8. zcat - zmore ............................................................................................... 17.9. bzip2 - bunzip2 .......................................................................................... 17.10. bzcat - bzmore ......................................................................................... 17.11. practice: basic Unix tools ........................................................................ 17.12. solution: basic Unix tools ........................................................................ 137 138 138 139 139 139 140 140 141 141 142 143 This chapter introduces commands to find or locate files and to compress files, together with other common tools that were not discussed before. While the tools discussed here are technically not considered filters, they can be used in pipes. 136
    • basic Unix tools 17.1. find The find command can be very useful at the start of a pipe to search for files. Here are some examples. You might want to add 2>/dev/null to the command lines to avoid cluttering your screen with error messages. Find all files in /etc and put the list in etcfiles.txt find /etc > etcfiles.txt Find all files of the entire system and put the list in allfiles.txt find / > allfiles.txt Find files that end in .conf in the current directory (and all subdirs). find . -name "*.conf" Find files of type file (not directory, pipe or etc.) that end in .conf. find . -type f -name "*.conf" Find files of type directory that end in .bak . find /data -type d -name "*.bak" Find files that are newer than file42.txt find . -newer file42.txt Find can also execute another command on every file found. This example will look for *.odf files and copy them to /backup/. find /data -name "*.odf" -exec cp {} /backup/ ; Find can also execute, after your confirmation, another command on every file found. This example will remove *.odf files if you approve of it for every file found. find /data -name "*.odf" -ok rm {} ; 137
    • basic Unix tools 17.2. locate The locate tool is very different from find in that it uses an index to locate files. This is a lot faster than traversing all the directories, but it also means that it is always outdated. If the index does not exist yet, then you have to create it (as root on Red Hat Enterprise Linux) with the updatedb command. [paul@RHEL4b ~]$ locate Samba warning: locate: could not open database: /var/lib/slocate/slocate.db:... warning: You need to run the 'updatedb' command (as root) to create th... Please have a look at /etc/updatedb.conf to enable the daily cron job. [paul@RHEL4b ~]$ updatedb fatal error: updatedb: You are not authorized to create a default sloc... [paul@RHEL4b ~]$ su Password: [root@RHEL4b ~]# updatedb [root@RHEL4b ~]# Most Linux distributions will schedule the updatedb to run once every day. 17.3. date The date command can display the date, time, time zone and more. paul@rhel55 ~$ date Sat Apr 17 12:44:30 CEST 2010 A date string can be customised to display the format of your choice. Check the man page for more options. paul@rhel55 ~$ date +'%A %d-%m-%Y' Saturday 17-04-2010 Time on any Unix is calculated in number of seconds since 1969 (the first second being the first second of the first of January 1970). Use date +%s to display Unix time in seconds. paul@rhel55 ~$ date +%s 1271501080 When will this seconds counter reach two thousand million ? paul@rhel55 ~$ date -d '1970-01-01 + 2000000000 seconds' Wed May 18 04:33:20 CEST 2033 138
    • basic Unix tools 17.4. cal The cal command displays the current month, with the current day highlighted. paul@rhel55 ~$ cal April 2010 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 You can select any month in the past or the future. paul@rhel55 February Su Mo Tu We 1 2 3 4 8 9 10 11 15 16 17 18 22 23 24 25 ~$ cal 2 1970 1970 Th Fr Sa 5 6 7 12 13 14 19 20 21 26 27 28 17.5. sleep The sleep command is sometimes used in scripts to wait a number of seconds. This example shows a five second sleep. paul@rhel55 ~$ sleep 5 paul@rhel55 ~$ 17.6. time The time command can display how long it takes to execute a command. The date command takes only a little time. paul@rhel55 ~$ time date Sat Apr 17 13:08:27 CEST 2010 real user sys 0m0.014s 0m0.008s 0m0.006s The sleep 5 command takes five real seconds to execute, but consumes little cpu time. paul@rhel55 ~$ time sleep 5 real user sys 0m5.018s 0m0.005s 0m0.011s 139
    • basic Unix tools This bzip2 command compresses a file and uses a lot of cpu time. paul@rhel55 ~$ time bzip2 text.txt real user sys 0m2.368s 0m0.847s 0m0.539s 17.7. gzip - gunzip Users never have enough disk space, so compression comes in handy. The gzip command can make files take up less space. paul@rhel55 ~$ ls -lh text.txt -rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt paul@rhel55 ~$ gzip text.txt paul@rhel55 ~$ ls -lh text.txt.gz -rw-rw-r-- 1 paul paul 760K Apr 17 13:11 text.txt.gz You can get the original back with gunzip. paul@rhel55 ~$ gunzip text.txt.gz paul@rhel55 ~$ ls -lh text.txt -rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt 17.8. zcat - zmore Text files that are compressed with gzip can be viewed with zcat and zmore. paul@rhel55 ~$ head -4 text.txt / /opt /opt/VBoxGuestAdditions-3.1.6 /opt/VBoxGuestAdditions-3.1.6/routines.sh paul@rhel55 ~$ gzip text.txt paul@rhel55 ~$ zcat text.txt.gz | head -4 / /opt /opt/VBoxGuestAdditions-3.1.6 /opt/VBoxGuestAdditions-3.1.6/routines.sh 140
    • basic Unix tools 17.9. bzip2 - bunzip2 Files can also be compressed with bzip2 which takes a little more time than gzip, but compresses better. paul@rhel55 ~$ bzip2 text.txt paul@rhel55 ~$ ls -lh text.txt.bz2 -rw-rw-r-- 1 paul paul 569K Apr 17 13:11 text.txt.bz2 Files can be uncompressed again with bunzip2. paul@rhel55 ~$ bunzip2 text.txt.bz2 paul@rhel55 ~$ ls -lh text.txt -rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt 17.10. bzcat - bzmore And in the same way bzcat and bzmore can display files compressed with bzip2. paul@rhel55 ~$ bzip2 text.txt paul@rhel55 ~$ bzcat text.txt.bz2 | head -4 / /opt /opt/VBoxGuestAdditions-3.1.6 /opt/VBoxGuestAdditions-3.1.6/routines.sh 141
    • basic Unix tools 17.11. practice: basic Unix tools 1. Explain the difference between these two commands. This question is very important. If you don't know the answer, then look back at the shell chapter. find /data -name "*.txt" find /data -name *.txt 2. Explain the difference between these two statements. Will they both work when there are 200 .odf files in /data ? How about when there are 2 million .odf files ? find /data -name "*.odf" > data_odf.txt find /data/*.odf > data_odf.txt 3. Write a find command that finds all files created after January 30th 2010. 4. Write a find command that finds all *.odf files created in September 2009. 5. Count the number of *.conf files in /etc and all its subdirs. 6. Two commands that do the same thing: copy *.odf files to /backup/ . What would be a reason to replace the first command with the second ? Again, this is an important question. cp -r /data/*.odf /backup/ find /data -name "*.odf" -exec cp {} /backup/ ; 7. Create a file called loctest.txt. Can you find this file with locate ? Why not ? How do you make locate find this file ? 8. Use find and -exec to rename all .htm files to .html. 9. Issue the date command. Now display the date in YYYY/MM/DD format. 10. Issue the cal command. Display a calendar of 1582 and 1752. Notice anything special ? 142
    • basic Unix tools 17.12. solution: basic Unix tools 1. Explain the difference between these two commands. This question is very important. If you don't know the answer, then look back at the shell chapter. find /data -name "*.txt" find /data -name *.txt When *.txt is quoted then the shell will not touch it. The find tool will look in the /data for all files ending in .txt. When *.txt is not quoted then the shell might expand this (when one or more files that ends in .txt exist in the current directory). The find might show a different result, or can result in a syntax error. 2. Explain the difference between these two statements. Will they both work when there are 200 .odf files in /data ? How about when there are 2 million .odf files ? find /data -name "*.odf" > data_odf.txt find /data/*.odf > data_odf.txt The first find will output all .odf filenames in /data and all subdirectories. The shell will redirect this to a file. The second find will output all files named .odf in /data and will also output all files that exist in directories named *.odf (in /data). With two million files the command line would be expanded beyond the maximum that the shell can accept. The last part of the command line would be lost. 3. Write a find command that finds all files created after January 30th 2010. touch -t 201001302359 marker_date find . -type f -newer marker_date There is another solution : find . -type f -newerat "20100130 23:59:59" 4. Write a find command that finds all *.odf files created in September 2009. touch -t 200908312359 marker_start touch -t 200910010000 marker_end find . -type f -name "*.odf" -newer marker_start ! -newer marker_end The exclamation mark ! -newer can be read as not newer. 5. Count the number of *.conf files in /etc and all its subdirs. find /etc -type f -name '*.conf' | wc -l 6. Two commands that do the same thing: copy *.odf files to /backup/ . What would be a reason to replace the first command with the second ? Again, this is an important question. cp -r /data/*.odf /backup/ 143
    • basic Unix tools find /data -name "*.odf" -exec cp {} /backup/ ; The first might fail when there are too many files to fit on one command line. 7. Create a file called loctest.txt. Can you find this file with locate ? Why not ? How do you make locate find this file ? You cannot locate this with locate because it is not yet in the index. updatedb 8. Use find and -exec to rename all .htm files to .html. paul@rhel55 ~$ find . -name '*.htm' ./one.htm ./two.htm paul@rhel55 ~$ find . -name '*.htm' -exec mv {} {}l ; paul@rhel55 ~$ find . -name '*.htm*' ./one.html ./two.html 9. Issue the date command. Now display the date in YYYY/MM/DD format. date +%Y/%m/%d 10. Issue the cal command. Display a calendar of 1582 and 1752. Notice anything special ? cal 1582 The calendars are different depending on the country. Check http://linux-training.be/ files/studentfiles/dates.txt 144
    • Part V. vi
    • Chapter 18. Introduction to vi Table of Contents 18.1. command mode and insert mode ............................................................... 147 18.2. start typing (a A i I o O) ........................................................................... 147 18.3. replace and delete a character (r x X) ........................................................ 148 18.4. undo and repeat (u .) .................................................................................. 148 18.5. cut, copy and paste a line (dd yy p P) ....................................................... 148 18.6. cut, copy and paste lines (3dd 2yy) ........................................................... 149 18.7. start and end of a line (0 or ^ and $) ......................................................... 149 18.8. join two lines (J) and more ........................................................................ 149 18.9. words (w b) ................................................................................................ 150 18.10. save (or not) and exit (:w :q :q! ) ............................................................. 150 18.11. Searching (/ ?) .......................................................................................... 151 18.12. replace all ( :1,$ s/foo/bar/g ) ................................................................... 151 18.13. reading files (:r :r !cmd) ........................................................................... 151 18.14. text buffers ............................................................................................... 152 18.15. multiple files ............................................................................................ 152 18.16. abbreviations ............................................................................................ 152 18.17. key mappings ........................................................................................... 153 18.18. setting options .......................................................................................... 153 18.19. practice: vi(m) .......................................................................................... 154 18.20. solution: vi(m) .......................................................................................... 155 The vi editor is installed on almost every Unix. Linux will very often install vim (vi improved) which is similar. Every system administrator should know vi(m), because it is an easy tool to solve problems. The vi editor is not intuitive, but once you get to know it, vi becomes a very powerful application. Most Linux distributions will include the vimtutor which is a 45 minute lesson in vi(m). 146
    • Introduction to vi 18.1. command mode and insert mode The vi editor starts in command mode. In command mode, you can type commands. Some commands will bring you into insert mode. In insert mode, you can type text. The escape key will return you to command mode. Table 18.1. getting to command mode key action Esc set vi(m) in command mode. 18.2. start typing (a A i I o O) The difference between a A i I o and O is the location where you can start typing. a will append after the current character and A will append at the end of the line. i will insert before the current character and I will insert at the beginning of the line. o will put you in a new line after the current line and O will put you in a new line before the current line. Table 18.2. switch to insert mode command action a start typing after the current character A start typing at the end of the current line i start typing before the current character I start typing at the start of the current line o start typing on a new line after the current line O start typing on a new line before the current line 147
    • Introduction to vi 18.3. replace and delete a character (r x X) When in command mode (it doesn't hurt to hit the escape key more than once) you can use the x key to delete the current character. The big X key (or shift x) will delete the character left of the cursor. Also when in command mode, you can use the r key to replace one single character. The r key will bring you in insert mode for just one key press, and will return you immediately to command mode. Table 18.3. replace and delete command action x delete the character below the cursor X delete the character before the cursor r replace the character below the cursor p paste after the cursor (here the last deleted character) xp switch two characters 18.4. undo and repeat (u .) When in command mode, you can undo your mistakes with u. You can do your mistakes twice with . (in other words, the . will repeat your last command). Table 18.4. undo and repeat command action u undo the last action . repeat the last action 18.5. cut, copy and paste a line (dd yy p P) When in command mode, dd will cut the current line. yy will copy the current line. You can paste the last copied or cut line after (p) or before (P) the current line. Table 18.5. cut, copy and paste a line command action dd cut the current line yy (yank yank) copy the current line p paste after the current line P paste before the current line 148
    • Introduction to vi 18.6. cut, copy and paste lines (3dd 2yy) When in command mode, before typing dd or yy, you can type a number to repeat the command a number of times. Thus, 5dd will cut 5 lines and 4yy will copy (yank) 4 lines. That last one will be noted by vi in the bottom left corner as "4 line yanked". Table 18.6. cut, copy and paste lines command action 3dd cut three lines 4yy copy four lines 18.7. start and end of a line (0 or ^ and $) When in command mode, the 0 and the caret ^ will bring you to the start of the current line, whereas the $ will put the cursor at the end of the current line. You can add 0 and $ to the d command, d0 will delete every character between the current character and the start of the line. Likewise d$ will delete everything from the current character till the end of the line. Similarly y0 and y$ will yank till start and end of the current line. Table 18.7. start and end of line command action 0 jump to start of current line ^ jump to start of current line $ jump to end of current line d0 delete until start of line d$ delete until end of line 18.8. join two lines (J) and more When in command mode, pressing J will append the next line to the current line. With yyp you duplicate a line and with ddp you switch two lines. Table 18.8. join two lines command action J join two lines yyp duplicate a line ddp switch two lines 149
    • Introduction to vi 18.9. words (w b) When in command mode, w will jump to the next word and b will move to the previous word. w and b can also be combined with d and y to copy and cut words (dw db yw yb). Table 18.9. words command action w forward one word b back one word 3w forward three words dw delete one word yw yank (copy) one word 5yb yank five words back 7dw delete seven words 18.10. save (or not) and exit (:w :q :q! ) Pressing the colon : will allow you to give instructions to vi (technically speaking, typing the colon will open the ex editor). :w will write (save) the file, :q will quit an unchanged file without saving, and :q! will quit vi discarding any changes. :wq will save and quit and is the same as typing ZZ in command mode. Table 18.10. save and exit vi command action :w :w fname :q save (write) save as fname quit :wq save and quit ZZ save and quit :q! quit (discarding your changes) :w! save (and write to non-writable file!) The last one is a bit special. With :w! vi will try to chmod the file to get write permission (this works when you are the owner) and will chmod it back when the write succeeds. This should always work when you are root (and the file system is writable). 150
    • Introduction to vi 18.11. Searching (/ ?) When in command mode typing / will allow you to search in vi for strings (can be a regular expression). Typing /foo will do a forward search for the string foo and typing ?bar will do a backward search for bar. Table 18.11. searching command action /string forward search for string ?string backward search for string n go to next occurrence of search string /^string forward search string at beginning of line /string$ forward search string at end of line /br[aeio]l /<he> search for bral brel bril and brol search for the word he (and not for here or the) 18.12. replace all ( :1,$ s/foo/bar/g ) To replace all occurrences of the string foo with bar, first switch to ex mode with : . Then tell vi which lines to use, for example 1,$ will do the replace all from the first to the last line. You can write 1,5 to only process the first five lines. The s/foo/bar/ g will replace all occurrences of foo with bar. Table 18.12. replace command action :4,8 s/foo/bar/g replace foo with bar on lines 4 to 8 :1,$ s/foo/bar/g replace foo with bar on all lines 18.13. reading files (:r :r !cmd) When in command mode, :r foo will read the file named foo, :r !foo will execute the command foo. The result will be put at the current location. Thus :r !ls will put a listing of the current directory in your text file. Table 18.13. read files and input command action :r fname (read) file fname and paste contents :r !cmd execute cmd and paste its output 151
    • Introduction to vi 18.14. text buffers There are 36 buffers in vi to store text. You can use them with the " character. Table 18.14. text buffers command action "add delete current line and put text in buffer a "g7yy copy seven lines into buffer g "ap paste from buffer a 18.15. multiple files You can edit multiple files with vi. Here are some tips. Table 18.15. multiple files command vi file1 file2 file3 :args action start editing three files lists files and marks active file :n start editing the next file :e toggle with last edited file :rew rewind file pointer to first file 18.16. abbreviations With :ab you can put abbreviations in vi. Use :una to undo the abbreviation. Table 18.16. abbreviations command :ab str long string :una str action abbreviate str to be 'long string' un-abbreviate str 152
    • Introduction to vi 18.17. key mappings Similarly to their abbreviations, you can use mappings with :map for command mode and :map! for insert mode. This example shows how to set the F6 function key to toggle between set number and set nonumber. The <bar> separates the two commands, set number! toggles the state and set number? reports the current state. :map <F6> :set number!<bar>set number?<CR> 18.18. setting options Some options that you can set in vim. :set number ( also try :se nu ) :set nonumber :syntax on :syntax off :set all (list all options) :set tabstop=8 :set tx (CR/LF style endings) :set notx You can set these options (and much more) in ~/.vimrc for vim or in ~/.exrc for standard vi. paul@barry:~$ cat ~/.vimrc set number set tabstop=8 set textwidth=78 map <F6> :set number!<bar>set number?<CR> paul@barry:~$ 153
    • Introduction to vi 18.19. practice: vi(m) 1. Start the vimtutor and do some or all of the exercises. You might need to run aptitude install vim on xubuntu. 2. What 3 key combination in command mode will duplicate the current line. 3. What 3 key combination in command mode will switch two lines' place (line five becomes line six and line six becomes line five). 4. What 2 key combination in command mode will switch a character's place with the next one. 5. vi can understand macro's. A macro can be recorded with q followed by the name of the macro. So qa will record the macro named a. Pressing q again will end the recording. You can recall the macro with @ followed by the name of the macro. Try this example: i 1 'Escape Key' qa yyp 'Ctrl a' q 5@a (Ctrl a will increase the number with one). 6. Copy /etc/passwd to your ~/passwd. Open the last one in vi and press Ctrl v. Use the arrow keys to select a Visual Block, you can copy this with y or delete it with d. Try pasting it. 7. What does dwwP do when you are at the beginning of a word in a sentence ? 154
    • Introduction to vi 18.20. solution: vi(m) 1. Start the vimtutor and do some or all of the exercises. You might need to run aptitude install vim on xubuntu. vimtutor 2. What 3 key combination in command mode will duplicate the current line. yyp 3. What 3 key combination in command mode will switch two lines' place (line five becomes line six and line six becomes line five). ddp 4. What 2 key combination in command mode will switch a character's place with the next one. xp 5. vi can understand macro's. A macro can be recorded with q followed by the name of the macro. So qa will record the macro named a. Pressing q again will end the recording. You can recall the macro with @ followed by the name of the macro. Try this example: i 1 'Escape Key' qa yyp 'Ctrl a' q 5@a (Ctrl a will increase the number with one). 6. Copy /etc/passwd to your ~/passwd. Open the last one in vi and press Ctrl v. Use the arrow keys to select a Visual Block, you can copy this with y or delete it with d. Try pasting it. cp /etc/passwd ~ vi passwd (press Ctrl-V) 7. What does dwwP do when you are at the beginning of a word in a sentence ? dwwP can switch the current word with the next word. 155
    • Part VI. scripting
    • Chapter 19. scripting introduction Table of Contents 19.1. prerequisites ............................................................................................... 19.2. hello world ................................................................................................. 19.3. she-bang ..................................................................................................... 19.4. comment ..................................................................................................... 19.5. variables ..................................................................................................... 19.6. sourcing a script ......................................................................................... 19.7. troubleshooting a script .............................................................................. 19.8. prevent setuid root spoofing ...................................................................... 19.9. practice: introduction to scripting .............................................................. 19.10. solution: introduction to scripting ............................................................ 158 158 158 159 159 159 160 160 161 162 Shells like bash and Korn have support for programming constructs that can be saved as scripts. These scripts in turn then become more shell commands. Many Linux commands are scripts. User profile scripts are run when a user logs on and init scripts are run when a daemon is stopped or started. This means that system administrators also need basic knowledge of scripting to understand how their servers and their applications are started, updated, upgraded, patched, maintained, configured and removed, and also to understand how a user environment is built. The goal of this chapter is to give you enough information to be able to read and understand scripts. Not to become a writer of complex scripts. 157
    • scripting introduction 19.1. prerequisites You should have read and understood part III shell expansion and part IV pipes and commands before starting this chapter. 19.2. hello world Just like in every programming course, we start with a simple hello_world script. The following script will output Hello World. echo Hello World After creating this simple script in vi or with echo, you'll have to chmod +x hello_world to make it executable. And unless you add the scripts directory to your path, you'll have to type the path to the script for the shell to be able to find it. [paul@RHEL4a [paul@RHEL4a [paul@RHEL4a Hello World [paul@RHEL4a ~]$ echo echo Hello World > hello_world ~]$ chmod +x hello_world ~]$ ./hello_world ~]$ 19.3. she-bang Let's expand our example a little further by putting #!/bin/bash on the first line of the script. The #! is called a she-bang (sometimes called sha-bang), where the shebang is the first two characters of the script. #!/bin/bash echo Hello World You can never be sure which shell a user is running. A script that works flawlessly in bash might not work in ksh, csh, or dash. To instruct a shell to run your script in a certain shell, you can start your script with a she-bang followed by the shell it is supposed to run in. This script will run in a bash shell. #!/bin/bash echo -n hello echo A bash subshell `echo -n hello` This script will run in a Korn shell (unless /bin/ksh is a hard link to /bin/bash). The /etc/shells file contains a list of shells on your system. #!/bin/ksh echo -n hello echo a Korn subshell `echo -n hello` 158
    • scripting introduction 19.4. comment Let's expand our example a little further by adding comment lines. #!/bin/bash # # Hello World Script # echo Hello World 19.5. variables Here is a simple example of a variable inside a script. #!/bin/bash # # simple variable in script # var1=4 echo var1 = $var1 Scripts can contain variables, but since scripts are run in their own shell, the variables do not survive the end of the script. [paul@RHEL4a ~]$ echo $var1 [paul@RHEL4a ~]$ ./vars var1 = 4 [paul@RHEL4a ~]$ echo $var1 [paul@RHEL4a ~]$ 19.6. sourcing a script Luckily, you can force a script to run in the same shell; this is called sourcing a script. [paul@RHEL4a ~]$ source ./vars var1 = 4 [paul@RHEL4a ~]$ echo $var1 4 [paul@RHEL4a ~]$ The above is identical to the below. [paul@RHEL4a ~]$ . ./vars var1 = 4 [paul@RHEL4a ~]$ echo $var1 4 [paul@RHEL4a ~]$ 159
    • scripting introduction 19.7. troubleshooting a script Another way to run a script in a separate shell is by typing bash with the name of the script as a parameter. paul@debian6~/test$ bash runme 42 Expanding this to bash -x allows you to see the commands that the shell is executing (after shell expansion). paul@debian6~/test$ bash -x runme + var4=42 + echo 42 42 paul@debian6~/test$ cat runme # the runme script var4=42 echo $var4 paul@debian6~/test$ Notice the absence of the commented (#) line, and the replacement of the variable before execution of echo. 19.8. prevent setuid root spoofing Some user may try to perform setuid based script root spoofing. This is a rare but possible attack. To improve script security and to avoid interpreter spoofing, you need to add -- after the #!/bin/bash, which disables further option processing so the shell will not accept any options. #!/bin/bash or #!/bin/bash -- Any arguments after the -- are treated as filenames and arguments. An argument of - is equivalent to --. 160
    • scripting introduction 19.9. practice: introduction to scripting 0. Give each script a different name, keep them for later! 1. Write a script that outputs the name of a city. 2. Make sure the script runs in the bash shell. 3. Make sure the script runs in the Korn shell. 4. Create a script that defines two variables, and outputs their value. 5. The previous script does not influence your current shell (the variables do not exist outside of the script). Now run the script so that it influences your current shell. 6. Is there a shorter way to source the script ? 7. Comment your scripts so that you know what they are doing. 161
    • scripting introduction 19.10. solution: introduction to scripting 0. Give each script a different name, keep them for later! 1. Write a script that outputs the name of a city. $ echo 'echo Antwerp' > first.bash $ chmod +x first.bash $ ./first.bash Antwerp 2. Make sure the script runs in the bash shell. $ cat first.bash #!/bin/bash echo Antwerp 3. Make sure the script runs in the Korn shell. $ cat first.bash #!/bin/ksh echo Antwerp Note that while first.bash will technically work as a Korn shell script, the name ending in .bash is confusing. 4. Create a script that defines two variables, and outputs their value. $ cat second.bash #!/bin/bash var33=300 var42=400 echo $var33 $var42 5. The previous script does not influence your current shell (the variables do not exist outside of the script). Now run the script so that it influences your current shell. source second.bash 6. Is there a shorter way to source the script ? . ./second.bash 7. Comment your scripts so that you know what they are doing. $ cat second.bash #!/bin/bash # script to test variables and sourcing # define two variables var33=300 var42=400 # output the value of these variables echo $var33 $var42 162
    • Chapter 20. scripting loops Table of Contents 20.1. 20.2. 20.3. 20.4. 20.5. 20.6. 20.7. 20.8. test [ ] ......................................................................................................... if then else ................................................................................................. if then elif .................................................................................................. for loop ....................................................................................................... while loop .................................................................................................. until loop .................................................................................................... practice: scripting tests and loops .............................................................. solution: scripting tests and loops .............................................................. 163 164 165 165 165 166 166 167 168
    • scripting loops 20.1. test [ ] The test command can test whether something is true or false. Let's start by testing whether 10 is greater than 55. [paul@RHEL4b ~]$ test 10 -gt 55 ; echo $? 1 [paul@RHEL4b ~]$ The test command returns 1 if the test fails. And as you see in the next screenshot, test returns 0 when a test succeeds. [paul@RHEL4b ~]$ test 56 -gt 55 ; echo $? 0 [paul@RHEL4b ~]$ If you prefer true and false, then write the test like this. [paul@RHEL4b ~]$ test 56 -gt 55 && echo true || echo false true [paul@RHEL4b ~]$ test 6 -gt 55 && echo true || echo false false The test command can also be written as square brackets, the screenshot below is identical to the one above. [paul@RHEL4b ~]$ [ 56 -gt 55 ] && echo true || echo false true [paul@RHEL4b ~]$ [ 6 -gt 55 ] && echo true || echo false false Below are some example tests. Take a look at man test to see more options for tests. [ [ [ [ [ [ [ [ [ [ [ -d foo ] -e bar ] '/etc' = $PWD ] $1 != 'secret' ] 55 -lt $bar ] $foo -ge 1000 ] "abc" < $bar ] -f foo ] -r bar ] foo -nt bar ] -o nounset ] Does the directory foo exist ? Does the file bar exist ? Is the string /etc equal to the variable $PWD ? Is the first parameter different from secret ? Is 55 less than the value of $bar ? Is the value of $foo greater or equal to 1000 ? Does abc sort before the value of $bar ? Is foo a regular file ? Is bar a readable file ? Is file foo newer than file bar ? Is the shell option nounset set ? Tests can be combined with logical AND and OR. paul@RHEL4b:~$ [ 66 -gt 55 -a 66 -lt 500 ] && echo true || echo false true paul@RHEL4b:~$ [ 66 -gt 55 -a 660 -lt 500 ] && echo true || echo false false paul@RHEL4b:~$ [ 66 -gt 55 -o 660 -lt 500 ] && echo true || echo false true 164
    • scripting loops 20.2. if then else The if then else construction is about choice. If a certain condition is met, then execute something, else execute something else. The example below tests whether a file exists, and if the file exists then a proper message is echoed. #!/bin/bash if [ -f isit.txt ] then echo isit.txt exists! else echo isit.txt not found! fi If we name the above script 'choice', then it executes like this. [paul@RHEL4a scripts]$ ./choice isit.txt not found! [paul@RHEL4a scripts]$ touch isit.txt [paul@RHEL4a scripts]$ ./choice isit.txt exists! [paul@RHEL4a scripts]$ 20.3. if then elif You can nest a new if inside an else with elif. This is a simple example. #!/bin/bash count=42 if [ $count -eq 42 ] then echo "42 is correct." elif [ $count -gt 42 ] then echo "Too much." else echo "Not enough." fi 20.4. for loop The example below shows the syntax of a classical for loop in bash. for i in 1 2 4 do echo $i done An example of a for loop combined with an embedded shell. #!/bin/ksh for counter in `seq 1 20` do echo counting from 1 to 20, now at $counter sleep 1 done 165
    • scripting loops The same example as above can be written without the embedded shell using the bash {from..to} shorthand. #!/bin/bash for counter in {1..20} do echo counting from 1 to 20, now at $counter sleep 1 done This for loop uses file globbing (from the shell expansion). Putting the instruction on the command line has identical functionality. kahlan@solexp11$ ls count.ksh go.ksh kahlan@solexp11$ for file in *.ksh ; do cp $file $file.backup ; done kahlan@solexp11$ ls count.ksh count.ksh.backup go.ksh go.ksh.backup 20.5. while loop Below a simple example of a while loop. i=100; while [ $i -ge 0 ] ; do echo Counting down, from 100 to 0, now at $i; let i--; done Endless loops can be made with while true or while : , where the colon is the equivalent of no operation in the Korn and bash shells. #!/bin/ksh # endless loop while : do echo hello sleep 1 done 20.6. until loop Below a simple example of an until loop. let i=100; until [ $i -le 0 ] ; do echo Counting down, from 100 to 1, now at $i; let i--; done 166
    • scripting loops 20.7. practice: scripting tests and loops 1. Write a script that uses a for loop to count from 3 to 7. 2. Write a script that uses a for loop to count from 1 to 17000. 3. Write a script that uses a while loop to count from 3 to 7. 4. Write a script that uses an until loop to count down from 8 to 4. 5. Write a script that counts the number of files ending in .txt in the current directory. 6. Wrap an if statement around the script so it is also correct when there are zero files ending in .txt. 167
    • scripting loops 20.8. solution: scripting tests and loops 1. Write a script that uses a for loop to count from 3 to 7. #!/bin/bash for i in 3 4 5 6 7 do echo Counting from 3 to 7, now at $i done 2. Write a script that uses a for loop to count from 1 to 17000. #!/bin/bash for i in `seq 1 17000` do echo Counting from 1 to 17000, now at $i done 3. Write a script that uses a while loop to count from 3 to 7. #!/bin/bash i=3 while [ $i -le 7 ] do echo Counting from 3 to 7, now at $i let i=i+1 done 4. Write a script that uses an until loop to count down from 8 to 4. #!/bin/bash i=8 until [ $i -lt 4 ] do echo Counting down from 8 to 4, now at $i let i=i-1 done 5. Write a script that counts the number of files ending in .txt in the current directory. #!/bin/bash let i=0 for file in *.txt do let i++ done echo "There are $i files ending in .txt" 6. Wrap an if statement around the script so it is also correct when there are zero files ending in .txt. #!/bin/bash ls *.txt > /dev/null 2>&1 if [ $? -ne 0 ] 168
    • scripting loops then echo "There are 0 files ending in .txt" else let i=0 for file in *.txt do let i++ done echo "There are $i files ending in .txt" fi 169
    • Chapter 21. scripting parameters Table of Contents 21.1. 21.2. 21.3. 21.4. 21.5. 21.6. 21.7. 21.8. script parameters ........................................................................................ shift through parameters ............................................................................ runtime input .............................................................................................. sourcing a config file ................................................................................. get script options with getopts ................................................................... get shell options with shopt ....................................................................... practice: parameters and options ................................................................ solution: parameters and options ............................................................... 170 171 172 172 173 174 175 176 177
    • scripting parameters 21.1. script parameters A bash shell script can have parameters. The numbering you see in the script below continues if you have more parameters. You also have special parameters containing the number of parameters, a string of all of them, and also the process id, and the last return code. The man page of bash has a full list. #!/bin/bash echo The first argument is $1 echo The second argument is $2 echo The third argument is $3 echo echo echo echo $ # ? * $$ $# $? $* PID of the script count arguments last return code all the arguments Below is the output of the script above in action. [paul@RHEL4a scripts]$ ./pars one two three The first argument is one The second argument is two The third argument is three $ 5610 PID of the script # 3 count arguments ? 0 last return code * one two three all the arguments Once more the same script, but with only two parameters. [paul@RHEL4a scripts]$ ./pars 1 2 The first argument is 1 The second argument is 2 The third argument is $ 5612 PID of the script # 2 count arguments ? 0 last return code * 1 2 all the arguments [paul@RHEL4a scripts]$ Here is another example, where we use $0. The $0 parameter contains the name of the script. paul@debian6~$ cat myname echo this script is called $0 paul@debian6~$ ./myname this script is called ./myname paul@debian6~$ mv myname test42 paul@debian6~$ ./test42 this script is called ./test42 171
    • scripting parameters 21.2. shift through parameters The shift statement can parse all parameters one by one. This is a sample script. kahlan@solexp11$ cat shift.ksh #!/bin/ksh if [ "$#" == "0" ] then echo You have to give at least one parameter. exit 1 fi while (( $# )) do echo You gave me $1 shift done Below is some sample output of the script above. kahlan@solexp11$ ./shift.ksh one You gave me one kahlan@solexp11$ ./shift.ksh one two three 1201 "33 42" You gave me one You gave me two You gave me three You gave me 1201 You gave me 33 42 kahlan@solexp11$ ./shift.ksh You have to give at least one parameter. 21.3. runtime input You can ask the user for input with the read command in a script. #!/bin/bash echo -n Enter a number: read number 172
    • scripting parameters 21.4. sourcing a config file The source (as seen in the shell chapters) can be used to source a configuration file. Below a sample configuration file for an application. [paul@RHEL4a scripts]$ cat myApp.conf # The config file of myApp # Enter the path here myAppPath=/var/myApp # Enter the number of quines here quines=5 And her an application that uses this file. [paul@RHEL4a scripts]$ cat myApp.bash #!/bin/bash # # Welcome to the myApp application # . ./myApp.conf echo There are $quines quines The running application can use the values inside the sourced configuration file. [paul@RHEL4a scripts]$ ./myApp.bash There are 5 quines [paul@RHEL4a scripts]$ 173
    • scripting parameters 21.5. get script options with getopts The getopts function allows you to parse options given to a command. The following script allows for any compination of the options a, f and z. kahlan@solexp11$ cat options.ksh #!/bin/ksh while getopts ":afz" option; do case $option in a) echo received -a ;; f) echo received -f ;; z) echo received -z ;; *) echo "invalid option -$OPTARG" ;; esac done This is sample output from the script above. First we use correct options, then we enter twice an invalid option. kahlan@solexp11$ ./options.ksh kahlan@solexp11$ ./options.ksh -af received -a received -f kahlan@solexp11$ ./options.ksh -zfg received -z received -f invalid option -g kahlan@solexp11$ ./options.ksh -a -b -z received -a invalid option -b received -z 174
    • scripting parameters You can also check for options that need an argument, as this example shows. kahlan@solexp11$ cat argoptions.ksh #!/bin/ksh while getopts ":af:z" option; do case $option in a) echo received -a ;; f) echo received -f with $OPTARG ;; z) echo received -z ;; :) echo "option -$OPTARG needs an argument" ;; *) echo "invalid option -$OPTARG" ;; esac done This is sample output from the script above. kahlan@solexp11$ ./argoptions.ksh -a -f hello -z received -a received -f with hello received -z kahlan@solexp11$ ./argoptions.ksh -zaf 42 received -z received -a received -f with 42 kahlan@solexp11$ ./argoptions.ksh -zf received -z option -f needs an argument 21.6. get shell options with shopt You can toggle the values of variables controlling optional shell behaviour with the shopt built-in shell command. The example below first verifies whether the cdspell option is set; it is not. The next shopt command sets the value, and the third shopt command verifies that the option really is set. You can now use minor spelling mistakes in the cd command. The man page of bash has a complete list of options. paul@laika:~$ 1 paul@laika:~$ paul@laika:~$ 0 paul@laika:~$ /etc shopt -q cdspell ; echo $? shopt -s cdspell shopt -q cdspell ; echo $? cd /Etc 175
    • scripting parameters 21.7. practice: parameters and options 1. Write a script that receives four parameters, and outputs them in reverse order. 2. Write a script that receives two parameters (two filenames) and outputs whether those files exist. 3. Write a script that asks for a filename. Verify existence of the file, then verify that you own the file, and whether it is writable. If not, then make it writable. 4. Make a configuration file for the previous script. Put a logging switch in the config file, logging means writing detailed output of everything the script does to a log file in /tmp. 176
    • scripting parameters 21.8. solution: parameters and options 1. Write a script that receives four parameters, and outputs them in reverse order. echo $4 $3 $2 $1 2. Write a script that receives two parameters (two filenames) and outputs whether those files exist. #!/bin/bash if [ -f $1 ] then echo $1 exists! else echo $1 not found! fi if [ -f $2 ] then echo $2 exists! else echo $2 not found! fi 3. Write a script that asks for a filename. Verify existence of the file, then verify that you own the file, and whether it is writable. If not, then make it writable. 4. Make a configuration file for the previous script. Put a logging switch in the config file, logging means writing detailed output of everything the script does to a log file in /tmp. 177
    • Chapter 22. more scripting Table of Contents 22.1. 22.2. 22.3. 22.4. 22.5. 22.6. 22.7. eval ............................................................................................................. (( )) ............................................................................................................. let ................................................................................................................ case ............................................................................................................. shell functions ............................................................................................ practice : more scripting ............................................................................ solution : more scripting ............................................................................ 178 179 179 180 181 182 183 184
    • more scripting 22.1. eval eval reads arguments as input to the shell (the resulting commands are executed). This allows using the value of a variable as a variable. paul@deb503:~/test42$ answer=42 paul@deb503:~/test42$ word=answer paul@deb503:~/test42$ eval x=$$word ; echo $x 42 Both in bash and Korn the arguments can be quoted. kahlan@solexp11$ answer=42 kahlan@solexp11$ word=answer kahlan@solexp11$ eval "y=$$word" ; echo $y 42 Sometimes the eval is needed to have correct parsing of arguments. Consider this example where the date command receives one parameter 1 week ago. paul@debian6~$ date --date="1 week ago" Thu Mar 8 21:36:25 CET 2012 When we set this command in a variable, then executing that variable fails unless we use eval. paul@debian6~$ lastweek='date --date="1 week ago"' paul@debian6~$ $lastweek date: extra operand `ago"' Try `date --help' for more information. paul@debian6~$ eval $lastweek Thu Mar 8 21:36:39 CET 2012 22.2. (( )) The (( )) allows for evaluation of numerical expressions. paul@deb503:~/test42$ true paul@deb503:~/test42$ false paul@deb503:~/test42$ paul@deb503:~/test42$ true paul@deb503:~/test42$ true paul@deb503:~/test42$ paul@deb503:~/test42$ false (( 42 > 33 )) && echo true || echo false (( 42 > 1201 )) && echo true || echo false var42=42 (( 42 == var42 )) && echo true || echo false (( 42 == $var42 )) && echo true || echo false var42=33 (( 42 == var42 )) && echo true || echo false 179
    • more scripting 22.3. let The let built-in shell function instructs the shell to perform an evaluation of arithmetic expressions. It will return 0 unless the last arithmetic expression evaluates to 0. [paul@RHEL4b 7 [paul@RHEL4b 20 [paul@RHEL4b 18 [paul@RHEL4b 30 ~]$ let x="3 + 4" ; echo $x ~]$ let x="10 + 100/10" ; echo $x ~]$ let x="10-2+100/10" ; echo $x ~]$ let x="10*2+100/10" ; echo $x The shell can also convert between different bases. [paul@RHEL4b 255 [paul@RHEL4b 192 [paul@RHEL4b 168 [paul@RHEL4b 56 [paul@RHEL4b 63 [paul@RHEL4b 192 ~]$ let x="0xFF" ; echo $x ~]$ let x="0xC0" ; echo $x ~]$ let x="0xA8" ; echo $x ~]$ let x="8#70" ; echo $x ~]$ let x="8#77" ; echo $x ~]$ let x="16#c0" ; echo $x There is a difference between assigning a variable directly, or using let to evaluate the arithmetic expressions (even if it is just assigning a value). kahlan@solexp11$ kahlan@solexp11$ 15 017 0x0f kahlan@solexp11$ kahlan@solexp11$ 15 15 15 dec=15 ; oct=017 ; hex=0x0f echo $dec $oct $hex let dec=15 ; let oct=017 ; let hex=0x0f echo $dec $oct $hex 180
    • more scripting 22.4. case You can sometimes simplify nested if statements with a case construct. [paul@RHEL4b ~]$ ./help What animal did you see ? lion You better start running fast! [paul@RHEL4b ~]$ ./help What animal did you see ? dog Don't worry, give it a cookie. [paul@RHEL4b ~]$ cat help #!/bin/bash # # Wild Animals Helpdesk Advice # echo -n "What animal did you see ? " read animal case $animal in "lion" | "tiger") echo "You better start running fast!" ;; "cat") echo "Let that mouse go..." ;; "dog") echo "Don't worry, give it a cookie." ;; "chicken" | "goose" | "duck" ) echo "Eggs for breakfast!" ;; "liger") echo "Approach and say 'Ah you big fluffy kitty...'." ;; "babelfish") echo "Did it fall out your ear ?" ;; *) echo "You discovered an unknown animal, name it!" ;; esac [paul@RHEL4b ~]$ 181
    • more scripting 22.5. shell functions Shell functions can be used to group commands in a logical way. kahlan@solexp11$ cat funcs.ksh #!/bin/ksh function greetings { echo Hello World! echo and hello to $USER to! } echo We will now call a function greetings echo The end This is sample output from this script with a function. kahlan@solexp11$ ./funcs.ksh We will now call a function Hello World! and hello to kahlan to! The end A shell function can also receive parameters. kahlan@solexp11$ cat addfunc.ksh #!/bin/ksh function plus { let result="$1 + $2" echo $1 + $2 = $result } plus 3 10 plus 20 13 plus 20 22 This script produces the following output. kahlan@solexp11$ ./addfunc.ksh 3 + 10 = 13 20 + 13 = 33 20 + 22 = 42 182
    • more scripting 22.6. practice : more scripting 1. Write a script that asks for two numbers, and outputs the sum and product (as shown here). Enter a number: 5 Enter another number: 2 Sum: Product: 5 + 2 = 7 5 x 2 = 10 2. Improve the previous script to test that the numbers are between 1 and 100, exit with an error if necessary. 3. Improve the previous script to congratulate the user if the sum equals the product. 4. Write a script with a case insensitive case statement, using the shopt nocasematch option. The nocasematch option is reset to the value it had before the scripts started. 5. If time permits (or if you are waiting for other students to finish this practice), take a look at linux system scripts in /etc/init.d and /etc/rc.d and try to understand them. Where does execution of a script start in /etc/init.d/samba ? There are also some hidden scripts in ~, we will discuss them later. 183
    • more scripting 22.7. solution : more scripting 1. Write a script that asks for two numbers, and outputs the sum and product (as shown here). Enter a number: 5 Enter another number: 2 Sum: Product: 5 + 2 = 7 5 x 2 = 10 #!/bin/bash echo -n "Enter a number : " read n1 echo -n "Enter another number : " read n2 let sum="$n1+$n2" let pro="$n1*$n2" echo -e "Sumt: $n1 + $n2 = $sum" echo -e "Productt: $n1 * $n2 = $pro" 2. Improve the previous script to test that the numbers are between 1 and 100, exit with an error if necessary. echo -n "Enter a number between 1 and 100 : " read n1 if [ $n1 -lt 1 -o $n1 -gt 100 ] then echo Wrong number... exit 1 fi 3. Improve the previous script to congratulate the user if the sum equals the product. if [ $sum -eq $pro ] then echo Congratulations $sum == $pro fi 4. Write a script with a case insensitive case statement, using the shopt nocasematch option. The nocasematch option is reset to the value it had before the scripts started. #!/bin/bash # # Wild Animals Case Insensitive Helpdesk Advice # if shopt -q nocasematch; then nocase=yes; else nocase=no; shopt -s nocasematch; fi echo -n "What animal did you see ? " read animal 184
    • more scripting case $animal in "lion" | "tiger") echo "You better start running fast!" ;; "cat") echo "Let that mouse go..." ;; "dog") echo "Don't worry, give it a cookie." ;; "chicken" | "goose" | "duck" ) echo "Eggs for breakfast!" ;; "liger") echo "Approach and say 'Ah you big fluffy kitty.'" ;; "babelfish") echo "Did it fall out your ear ?" ;; *) echo "You discovered an unknown animal, name it!" ;; esac if [ nocase = yes ] ; then shopt -s nocasematch; else shopt -u nocasematch; fi 5. If time permits (or if you are waiting for other students to finish this practice), take a look at linux system scripts in /etc/init.d and /etc/rc.d and try to understand them. Where does execution of a script start in /etc/init.d/samba ? There are also some hidden scripts in ~, we will discuss them later. 185
    • Part VII. local user management
    • Chapter 23. users Table of Contents 23.1. identify yourself ......................................................................................... 23.2. users ........................................................................................................... 23.3. passwords ................................................................................................... 23.4. home directories ......................................................................................... 23.5. user shell .................................................................................................... 23.6. switch users with su ................................................................................... 23.7. run a program as another user ................................................................... 23.8. practice: users ............................................................................................ 23.9. solution: users ............................................................................................ 23.10. shell environment ..................................................................................... 187 188 189 191 196 197 198 199 201 202 204
    • users 23.1. identify yourself whoami The whoami command tells you your username. [root@RHEL5 ~]# whoami root [root@RHEL5 ~]# su - paul [paul@RHEL5 ~]$ whoami paul who The who command will give you information about who is logged on the system. [paul@RHEL5 ~]$ who root tty1 sandra pts/0 paul pts/1 2008-06-24 13:24 2008-06-24 14:05 (192.168.1.34) 2008-06-24 16:23 (192.168.1.37) who am i With who am i the who command will display only the line pointing to your current session. [paul@RHEL5 ~]$ who am i paul pts/1 2008-06-24 16:23 (192.168.1.34) w The w command shows you who is logged on and what they are doing. $ w 05:13:36 up 3 min, 4 users, load average: 0.48, 0.72, 0.33 USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT root tty1 05:11 2.00s 0.32s 0.27s find / -name shad inge pts/0 192.168.1.33 05:12 0.00s 0.02s 0.02s -ksh paul pts/2 192.168.1.34 05:13 25.00s 0.07s 0.04s top id The id command will give you your user id, primary group id, and a list of the groups that you belong to. root@laika:~# id uid=0(root) gid=0(root) groups=0(root) root@laika:~# su - brel brel@laika:~$ id uid=1001(brel) gid=1001(brel) groups=1001(brel),1008(chanson),11578(wolf) 188
    • users 23.2. users user management User management on any Unix can be done in three complimentary ways. You can use the graphical tools provided by your distribution. These tools have a look and feel that depends on the distribution. If you are a novice Linux user on your home system, then use the graphical tool that is provided by your distribution. This will make sure that you do not run into problems. Another option is to use command line tools like useradd, usermod, gpasswd, passwd and others. Server administrators are likely to use these tools, since they are familiar and very similar across many different distributions. This chapter will focus on these command line tools. A third and rather extremist way is to edit the local configuration files directly using vi (or vipw/vigr). Do not attempt this as a novice on production systems! /etc/passwd The local user database on Linux (and on most Unixes) is /etc/passwd. [root@RHEL5 ~]# tail /etc/passwd inge:x:518:524:art dealer:/home/inge:/bin/ksh ann:x:519:525:flute player:/home/ann:/bin/bash frederik:x:520:526:rubius poet:/home/frederik:/bin/bash steven:x:521:527:roman emperor:/home/steven:/bin/bash pascale:x:522:528:artist:/home/pascale:/bin/ksh geert:x:524:530:kernel developer:/home/geert:/bin/bash wim:x:525:531:master damuti:/home/wim:/bin/bash sandra:x:526:532:radish stresser:/home/sandra:/bin/bash annelies:x:527:533:sword fighter:/home/annelies:/bin/bash laura:x:528:534:art dealer:/home/laura:/bin/ksh As you can see, this file contains seven columns separated by a colon. The columns contain the username, an x, the user id, the primary group id, a description, the name of the home directory, and the login shell. root The root user also called the superuser is the most powerful account on your Linux system. This user can do almost anything, including the creation of other users. The root user always has userid 0 (regardless of the name of the account). [root@RHEL5 ~]# head -1 /etc/passwd root:x:0:0:root:/root:/bin/bash 189
    • users useradd You can add users with the useradd command. The example below shows how to add a user named yanina (last parameter) and at the same time forcing the creation of the home directory (-m), setting the name of the home directory (-d), and setting a description (-c). [root@RHEL5 ~]# useradd -m -d /home/yanina -c "yanina wickmayer" yanina [root@RHEL5 ~]# tail -1 /etc/passwd yanina:x:529:529:yanina wickmayer:/home/yanina:/bin/bash The user named yanina received userid 529 and primary group id 529. /etc/default/useradd Both Red Hat Enterprise Linux and Debian/Ubuntu have a file called /etc/default/ useradd that contains some default user options. Besides using cat to display this file, you can also use useradd -D. [root@RHEL4 ~]# useradd -D GROUP=100 HOME=/home INACTIVE=-1 EXPIRE= SHELL=/bin/bash SKEL=/etc/skel userdel You can delete the user yanina with userdel. The -r option of userdel will also remove the home directory. [root@RHEL5 ~]# userdel -r yanina usermod You can modify the properties of a user with the usermod command. This example uses usermod to change the description of the user harry. [root@RHEL4 ~]# tail -1 /etc/passwd harry:x:516:520:harry potter:/home/harry:/bin/bash [root@RHEL4 ~]# usermod -c 'wizard' harry [root@RHEL4 ~]# tail -1 /etc/passwd harry:x:516:520:wizard:/home/harry:/bin/bash 190
    • users 23.3. passwords passwd Passwords of users can be set with the passwd command. Users will have to provide their old password before twice entering the new one. [harry@RHEL4 ~]$ passwd Changing password for user harry. Changing password for harry (current) UNIX password: New UNIX password: BAD PASSWORD: it's WAY too short New UNIX password: Retype new UNIX password: passwd: all authentication tokens updated successfully. [harry@RHEL4 ~]$ As you can see, the passwd tool will do some basic verification to prevent users from using too simple passwords. The root user does not have to follow these rules (there will be a warning though). The root user also does not have to provide the old password before entering the new password twice. /etc/shadow User passwords are encrypted and kept in /etc/shadow. The /etc/shadow file is read only and can only be read by root. We will see in the file permissions section how it is possible for users to change their password. For now, you will have to know that users can change their password with the /usr/bin/passwd command. [root@RHEL5 ~]# tail /etc/shadow inge:$1$yWMSimOV$YsYvcVKqByFVYLKnU3ncd0:14054:0:99999:7::: ann:!!:14054:0:99999:7::: frederik:!!:14054:0:99999:7::: steven:!!:14054:0:99999:7::: pascale:!!:14054:0:99999:7::: geert:!!:14054:0:99999:7::: wim:!!:14054:0:99999:7::: sandra:!!:14054:0:99999:7::: annelies:!!:14054:0:99999:7::: laura:$1$Tvby1Kpa$lL.WzgobujUS3LClIRmdv1:14054:0:99999:7::: The /etc/shadow file contains nine colon separated columns. The nine fields contain (from left to right) the user name, the encrypted password (note that only inge and laura have an encrypted password), the day the password was last changed (day 1 is January 1, 1970), number of days the password must be left unchanged, password expiry day, warning number of days before password expiry, number of days after expiry before disabling the account, and the day the account was disabled (again, since 1970). The last field has no meaning yet. 191
    • users password encryption encryption with passwd Passwords are stored in an encrypted format. This encryption is done by the crypt function. The easiest (and recommended) way to add a user with a password to the system is to add the user with the useradd -m user command, and then set the user's password with passwd. [root@RHEL4 ~]# useradd -m xavier [root@RHEL4 ~]# passwd xavier Changing password for user xavier. New UNIX password: Retype new UNIX password: passwd: all authentication tokens updated successfully. [root@RHEL4 ~]# encryption with openssl Another way to create users with a password is to use the -p option of useradd, but that option requires an encrypted password. You can generate this encrypted password with the openssl passwd command. [root@RHEL4 ~]# openssl passwd stargate ZZNX16QZVgUQg [root@RHEL4 ~]# useradd -m -p ZZNX16QZVgUQg mohamed encryption with crypt A third option is to create your own C program using the crypt function, and compile this into a command. [paul@laika ~]$ cat MyCrypt.c #include <stdio.h> #define __USE_XOPEN #include <unistd.h> int main(int argc, char** argv) { if(argc==3) { printf("%sn", crypt(argv[1],argv[2])); } else { printf("Usage: MyCrypt $password $saltn" ); } return 0; } 192
    • users This little program can be compiled with gcc like this. [paul@laika ~]$ gcc MyCrypt.c -o MyCrypt -lcrypt To use it, we need to give two parameters to MyCript. The first is the unencrypted password, the second is the salt. The salt is used to perturb the encryption algorithm in one of 4096 different ways. This variation prevents two users with the same password from having the same entry in /etc/shadow. paul@laika:~$ 12L4FoTS3/k9U paul@laika:~$ 01Y.yPnlQ6R.Y paul@laika:~$ 330asFUbzgVeg paul@laika:~$ 42XFxoT4R75gk ./MyCrypt stargate 12 ./MyCrypt stargate 01 ./MyCrypt stargate 33 ./MyCrypt stargate 42 Did you notice that the first two characters of the password are the salt? The standard output of the crypt function is using the DES algorithm which is old and can be cracked in minutes. A better method is to use md5 passwords which can be recognized by a salt starting with $1$. paul@laika:~$ ./MyCrypt stargate $1$12$xUIQ4116Us.Q5Osc2Khbm1 paul@laika:~$ ./MyCrypt stargate $1$01$yNs8brjp4b4TEw.v9/IlJ/ paul@laika:~$ ./MyCrypt stargate $1$33$tLh/Ldy2wskdKAJR.Ph4M0 paul@laika:~$ ./MyCrypt stargate $1$42$Hb3nvP0KwHSQ7fQmIlY7R. '$1$12' '$1$01' '$1$33' '$1$42' The md5 salt can be up to eight characters long. The salt is displayed in /etc/shadow between the second and third $, so never use the password as the salt! paul@laika:~$ ./MyCrypt stargate '$1$stargate' $1$stargate$qqxoLqiSVNvGr5ybMxEVM1 193
    • users password defaults /etc/login.defs The /etc/login.defs file contains some default settings for user passwords like password aging and length settings. (You will also find the numerical limits of user ids and group ids and whether or not a home directory should be created by default). [root@RHEL4 ~]# grep -i pass /etc/login.defs # Password aging controls: # PASS_MAX_DAYS Maximum number of days a password may be used. # PASS_MIN_DAYS Minimum number of days allowed between password changes. # PASS_MIN_LEN Minimum acceptable password length. # PASS_WARN_AGE Number of days warning given before a password expires. PASS_MAX_DAYS 99999 PASS_MIN_DAYS 0 PASS_MIN_LEN 5 PASS_WARN_AGE 7 chage The chage command can be used to set an expiration date for a user account (-E), set a minimum (-m) and maximum (-M) password age, a password expiration date, and set the number of warning days before the password expiration date. Much of this functionality is also available from the passwd command. The -l option of chage will list these settings for a user. [root@RHEL4 ~]# chage -l harry Minimum: 0 Maximum: 99999 Warning: 7 Inactive: -1 Last Change: Jul 23, 2007 Password Expires: Never Password Inactive: Never Account Expires: Never [root@RHEL4 ~]# disabling a password Passwords in /etc/shadow cannot begin with an exclamation mark. When the second field in /etc/passwd starts with an exclamation mark, then the password can not be used. Using this feature is often called locking, disabling, or suspending a user account. Besides vi (or vipw) you can also accomplish this with usermod. The first line in the next screenshot will disable the password of user harry, making it impossible for harry to authenticate using this password. 194
    • users [root@RHEL4 ~]# usermod -L harry [root@RHEL4 ~]# tail -1 /etc/shadow harry:!$1$143TO9IZ$RLm/FpQkpDrV4/Tkhku5e1:13717:0:99999:7::: The root user (and users with sudo rights on su) still will be able to su to harry (because the password is not needed here). Also note that harry will still be able to login if he has set up passwordless ssh! [root@RHEL4 ~]# su - harry [harry@RHEL4 ~]$ You can unlock the account again with usermod -U. Watch out for tiny differences in the command line options of passwd, usermod, and useradd on different distributions! Verify the local files when using features like "disabling, suspending, or locking" users and passwords! editing local files If you still want to manually edit the /etc/passwd or /etc/shadow, after knowing these commands for password management, then use vipw instead of vi(m) directly. The vipw tool will do proper locking of the file. [root@RHEL5 ~]# vipw /etc/passwd vipw: the password file is busy (/etc/ptmp present) 195
    • users 23.4. home directories creating home directories The easiest way to create a home directory is to supply the -m option with useradd (it is likely set as a default option on Linux). A less easy way is to create a home directory manually with mkdir which also requires setting the owner and the permissions on the directory with chmod and chown (both commands are discussed in detail in another chapter). [root@RHEL5 ~]# mkdir /home/laura [root@RHEL5 ~]# chown laura:laura /home/laura [root@RHEL5 ~]# chmod 700 /home/laura [root@RHEL5 ~]# ls -ld /home/laura/ drwx------ 2 laura laura 4096 Jun 24 15:17 /home/laura/ /etc/skel/ When using useradd the -m option, the /etc/skel/ directory is copied to the newly created home directory. The /etc/skel/ directory contains some (usually hidden) files that contain profile settings and default values for applications. In this way /etc/skel/ serves as a default home directory and as a default user profile. [root@RHEL5 ~]# ls total 48 drwxr-xr-x 2 root drwxr-xr-x 97 root -rw-r--r-- 1 root -rw-r--r-- 1 root -rw-r--r-- 1 root -la /etc/skel/ root 4096 Apr 1 00:11 . root 12288 Jun 24 15:36 .. root 24 Jul 12 2006 .bash_logout root 176 Jul 12 2006 .bash_profile root 124 Jul 12 2006 .bashrc deleting home directories The -r option of userdel will make sure that the home directory is deleted together with the user account. [root@RHEL5 ~]# ls -ld /home/wim/ drwx------ 2 wim wim 4096 Jun 24 15:19 /home/wim/ [root@RHEL5 ~]# userdel -r wim [root@RHEL5 ~]# ls -ld /home/wim/ ls: /home/wim/: No such file or directory 196
    • users 23.5. user shell login shell The /etc/passwd file specifies the login shell for the user. In the screenshot below you can see that user annelies will log in with the /bin/bash shell, and user laura with the /bin/ksh shell. [root@RHEL5 ~]# tail -2 /etc/passwd annelies:x:527:533:sword fighter:/home/annelies:/bin/bash laura:x:528:534:art dealer:/home/laura:/bin/ksh You can use the usermod command to change the shell for a user. [root@RHEL5 ~]# usermod -s /bin/bash laura [root@RHEL5 ~]# tail -1 /etc/passwd laura:x:528:534:art dealer:/home/laura:/bin/bash chsh Users can change their login shell with the chsh command. First, user harry obtains a list of available shells (he could also have done a cat /etc/shells) and then changes his login shell to the Korn shell (/bin/ksh). At the next login, harry will default into ksh instead of bash. [harry@RHEL4 ~]$ chsh -l /bin/sh /bin/bash /sbin/nologin /bin/ash /bin/bsh /bin/ksh /usr/bin/ksh /usr/bin/pdksh /bin/tcsh /bin/csh /bin/zsh [harry@RHEL4 ~]$ chsh -s /bin/ksh Changing shell for harry. Password: Shell changed. [harry@RHEL4 ~]$ 197
    • users 23.6. switch users with su su to another user The su command allows a user to run a shell as another user. [paul@RHEL4b ~]$ su harry Password: [harry@RHEL4b paul]$ su to root Yes you can alsu su to become root, when you know the root password. [harry@RHEL4b paul]$ su root Password: [root@RHEL4b paul]# su as root Unless you are logged in as root, running a shell as another user requires that you know the password of that user. The root user can become any user without knowing the user's password. [root@RHEL4b paul]# su serena [serena@RHEL4b paul]$ su - $username By default, the su command maintains the same shell environment. To become another user and also get the target user's environment, issue the su - command followed by the target username. [paul@RHEL4b ~]$ su - harry Password: [harry@RHEL4b ~]$ su When no username is provided to su or su -, the command will assume root is the target. [harry@RHEL4b ~]$ su Password: [root@RHEL4b ~]# 198
    • users 23.7. run a program as another user about sudo The sudo program allows a user to start a program with the credentials of another user. Before this works, the system administrator has to set up the /etc/sudoers file. This can be useful to delegate administrative tasks to another user (without giving the root password). The screenshot below shows the usage of sudo. User paul received the right to run useradd with the credentials of root. This allows paul to create new users on the system without becoming root and without knowing the root password. paul@laika:~$ useradd -m inge useradd: unable to lock password file paul@laika:~$ sudo useradd -m inge [sudo] password for paul: paul@laika:~$ setuid on sudo The sudo binary has the setuid bit set, so any user can run it with the effective userid of root. paul@laika:~$ ls -l `which sudo` -rwsr-xr-x 2 root root 107872 2008-05-15 02:41 /usr/bin/sudo paul@laika:~$ visudo Check the man page of visudo before playing with the /etc/sudoers file. sudo su On some linux systems like Ubuntu and Kubuntu, the root user does not have a password set. This means that it is not possible to login as root (extra security). To perform tasks as root, the first user is given all sudo rights via the /etc/sudoers. In fact all users that are members of the admin group can use sudo to run all commands as root. root@laika:~# grep admin /etc/sudoers # Members of the admin group may gain root privileges %admin ALL=(ALL) ALL 199
    • users The end result of this is that the user can type sudo su - and become root without having to enter the root password. The sudo command does require you to enter your own password. Thus the password prompt in the screenshot below is for sudo, not for su. paul@laika:~$ sudo su Password: root@laika:~# 200
    • users 23.8. practice: users 1. Create the users Serena Williams, Venus Williams and Justine Henin, all of them with password set to stargate, with username (lower case!) as their first name, and their full name in the comment. Verify that the users and their home directory are properly created. 2. Create a user called kornuser, give him the Korn shell (/bin/ksh) as his default shell. Log on with this user (on a command line or in a tty). 3. Create a user named einstime without home directory, give him /bin/date as his default logon shell. What happens when you log on with this user ? Can you think of a useful real world example for changing a user's login shell to an application ? 4. Try the commands who, whoami, who am i, w, id, echo $USER $UID . 5a. Lock the venus user account with usermod. 5b. Use passwd -d to disable the serena password. Verify the serena line in /etc/ shadow before and after disabling. 5c. What is the difference between locking a user account and disabling a user account's password ? 6. As root change the password of einstime to stargate. 7. Now try changing the password of serena to serena as serena. 8. Make sure every new user needs to change his password every 10 days. 9. Set the warning number of days to four for the kornuser. 10a. Set the password of two separate users to stargate. Look at the encrypted stargate's in /etc/shadow and explain. 10b. Take a backup as root of /etc/shadow. Use vi to copy an encrypted stargate to another user. Can this other user now log on with stargate as a password ? 11. Put a file in the skeleton directory and check whether it is copied to user's home directory. When is the skeleton directory copied ? 12. Why use vipw instead of vi ? What could be the problem when using vi or vim ? 13. Use chsh to list all shells, and compare to cat /etc/shells. Change your login shell to the Korn shell, log out and back in. Now change back to bash. 14. Which useradd option allows you to name a home directory ? 15. How can you see whether the password of user harry is locked or unlocked ? Give a solution with grep and a solution with passwd. 201
    • users 23.9. solution: users 1. Create the users Serena Williams, Venus Williams and Justine Henin, all of them with password set to stargate, with username (lower case) as their first name, and their full name in the comment. Verify that the users and their home directory are properly created. useradd -m -c "Serena Williams" serena ; passwd serena useradd -m -c "Venus Williams" venus ; passwd venus useradd -m -c "Justine Henin" justine ; passwd justine tail /etc/passwd ; tail /etc/shadow ; ls /home Keep user logon names in lowercase! 2. Create a user called kornuser, give him the Korn shell (/bin/ksh) as his default shell. Log on with this user (on a command line or in a tty). useradd -s /bin/ksh kornuser ; passwd kornuser 3. Create a user named einstime without home directory, give him /bin/date as his default logon shell. What happens when you log on with this user ? Can you think of a useful real world example for changing a user's login shell to an application ? useradd -s /bin/date einstime ; passwd einstime It can be useful when users need to access only one application on the server. Just logging on opens the application for them, and closing the application automatically logs them off. 4. Try the commands who, whoami, who am i, w, id, echo $USER $UID . who ; whoami ; who am i ; w ; id ; echo $USER $UID 5a. Lock the venus user account with usermod. usermod -L venus 5b. Use passwd -d to disable the serena password. Verify the serena line in /etc/ shadow before and after disabling. grep serena /etc/shadow; passwd -d serena ; grep serena /etc/shadow 5c. What is the difference between locking a user account and disabling a user account's password ? Locking will prevent the user from logging on to the system with his password (by putting a ! in front of the password in /etc/shadow). Disabling with passwd will erase the password from /etc/shadow. 6. As root change the password of einstime to stargate. Log on as root and type: passwd einstime 7. Now try changing the password of serena to serena as serena. log on as serena, then execute: passwd serena... it should fail! 202
    • users 8. Make sure every new user needs to change his password every 10 days. For an existing user: chage -M 10 serena For all new users: vi /etc/login.defs (and change PASS_MAX_DAYS to 10) 9. Set the warning number of days to four for the kornuser. chage -W 4 kornuser 10a. Set the password of two separate users to stargate. Look at the encrypted stargate's in /etc/shadow and explain. If you used passwd, then the salt will be different for the two encrypted passwords. 10b. Take a backup as root of /etc/shadow. Use vi to copy an encrypted stargate to another user. Can this other user now log on with stargate as a password ? Yes. 11. Put a file in the skeleton directory and check whether it is copied to user's home directory. When is the skeleton directory copied ? When you create a user account with a new home directory. 12. Why use vipw instead of vi ? What could be the problem when using vi or vim ? vipw will give a warning when someone else is already using that file. 13. Use chsh to list all shells, and compare to cat /etc/shells. Change your login shell to the Korn shell, log out and back in. Now change back to bash. On Red Hat Enterprise Linux: chsh -l On Debian/Ubuntu: cat /etc/shells 14. Which useradd option allows you to name a home directory ? -d 15. How can you see whether the password of user harry is locked or unlocked ? Give a solution with grep and a solution with passwd. grep harry /etc/shadow passwd -S harry 203
    • users 23.10. shell environment It is nice to have these preset and custom aliases and variables, but where do they all come from ? The shell uses a number of startup files that are checked (and executed) whenever the shell is invoked. What follows is an overview of startup scripts. /etc/profile Both the bash and the ksh shell will verify the existence of /etc/profile and execute it if it exists. When reading this script, you might notice (at least on Debian Lenny and on Red Hat Enterprise Linux 5) that it builds the PATH environment variable. The script might also change the PS1 variable, set the HOSTNAME and execute even more scripts like /etc/inputrc You can use this script to set aliases and variables for every user on the system. ~/.bash_profile When this file exists in the users home directory, then bash will execute it. On Debian Linux it does not exist by default. RHEL5 uses a brief ~/.bash_profile where it checks for the existence of ~/.bashrc and then executes it. It also adds $HOME/bin to the $PATH variable. [serena@rhel53 ~]$ cat .bash_profile # .bash_profile # Get the aliases and functions if [ -f ~/.bashrc ]; then . ~/.bashrc fi # User specific environment and startup programs PATH=$PATH:$HOME/bin export PATH ~/.bash_login When .bash_profile does not exist, then bash will check for ~/.bash_login and execute it. Neither Debian nor Red Hat have this file by default. 204
    • users ~/.profile When neither ~/.bash_profile and ~/.bash_login exist, then bash will verify the existence of ~/.profile and execute it. This file does not exist by default on Red Hat. On Debian this script can execute ~/.bashrc and will add $HOME/bin to the $PATH variable. serena@deb503:~$ tail -12 .profile # if running bash if [ -n "$BASH_VERSION" ]; then # include .bashrc if it exists if [ -f "$HOME/.bashrc" ]; then . "$HOME/.bashrc" fi fi # set PATH so it includes user's private bin if it exists if [ -d "$HOME/bin" ] ; then PATH="$HOME/bin:$PATH" fi ~/.bashrc As seen in the previous points, the ~/.bashrc script might be executed by other scripts. Let us take a look at what it does by default. Red Hat uses a very simple ~/.bashrc, checking for /etc/bashrc and executing it. It also leaves room for custom aliases and functions. [serena@rhel53 ~]$ more .bashrc # .bashrc # Source global definitions if [ -f /etc/bashrc ]; then . /etc/bashrc fi # User specific aliases and functions On Debian this script is quite a bit longer and configures $PS1, some history variables and a number af active and inactive aliases. serena@deb503:~$ ls -l .bashrc -rw-r--r-- 1 serena serena 3116 2008-05-12 21:02 .bashrc 205
    • users ~/.bash_logout When exiting bash, it can execute ~/.bash_logout. Debian and Red Hat both use this opportunity to clear the screen. serena@deb503:~$ cat .bash_logout # ~/.bash_logout: executed by bash(1) when login shell exits. # when leaving the console clear the screen to increase privacy if [ "$SHLVL" = 1 ]; then [ -x /usr/bin/clear_console ] && /usr/bin/clear_console -q fi [serena@rhel53 ~]$ cat .bash_logout # ~/.bash_logout /usr/bin/clear Debian overview Below is a table overview of when Debian is running any of these bash startup scripts. Table 23.1. Debian User Environment script su su - ssh gdm ~./bashrc no yes yes yes ~/.profile no yes yes yes /etc/profile no yes yes yes /etc/bash.bashrc yes no no yes RHEL5 overview Below is a table overview of when Red Hat Enterprise Linux 5 is running any of these bash startup scripts. Table 23.2. Red Hat User Environment script su su - ssh gdm ~./bashrc yes yes yes yes ~/.bash_profile no yes yes yes /etc/profile no yes yes yes /etc/bashrc yes yes yes yes 206
    • Chapter 24. groups Table of Contents 24.1. about groups ............................................................................................... 24.2. groupadd ..................................................................................................... 24.3. /etc/group .................................................................................................... 24.4. usermod ...................................................................................................... 24.5. groupmod ................................................................................................... 24.6. groupdel ...................................................................................................... 24.7. groups ......................................................................................................... 24.8. gpasswd ...................................................................................................... 24.9. vigr ............................................................................................................. 24.10. practice: groups ........................................................................................ 24.11. solution: groups ........................................................................................ 207 208 208 208 209 209 209 209 210 210 211 212
    • groups 24.1. about groups Users can be listed in groups. Groups allow you to set permissions on the group level instead of having to set permissions for every individual user. Every Unix or Linux distribution will have a graphical tool to manage groups. Novice users are advised to use this graphical tool. More experienced users can use command line tools to manage users, but be careful: Some distributions do not allow the mixed use of GUI and CLI tools to manage groups (YaST in Novell Suse). Senior administrators can edit the relevant files directly with vi or vigr. 24.2. groupadd Groups can be created with the groupadd command. The example below shows the creation of five (empty) groups. root@laika:~# root@laika:~# root@laika:~# root@laika:~# root@laika:~# groupadd groupadd groupadd groupadd groupadd tennis football snooker formula1 salsa 24.3. /etc/group Users can be a member of several groups. Group membership is defined by the /etc/ group file. root@laika:~# tail -5 /etc/group tennis:x:1006: football:x:1007: snooker:x:1008: formula1:x:1009: salsa:x:1010: root@laika:~# The first field is the group's name. The second field is the group's (encrypted) password (can be empty). The third field is the group identification or GID. The fourth field is the list of members, these groups have no members. 208
    • groups 24.4. usermod Group membership can be modified with the useradd or usermod command. root@laika:~# usermod -a -G tennis inge root@laika:~# usermod -a -G tennis katrien root@laika:~# usermod -a -G salsa katrien root@laika:~# usermod -a -G snooker sandra root@laika:~# usermod -a -G formula1 annelies root@laika:~# tail -5 /etc/group tennis:x:1006:inge,katrien football:x:1007: snooker:x:1008:sandra formula1:x:1009:annelies salsa:x:1010:katrien root@laika:~# Be careful when using usermod to add users to groups. By default, the usermod command will remove the user from every group of which he is a member if the group is not listed in the command! Using the -a (append) switch prevents this behaviour. 24.5. groupmod You can change the group name with the groupmod command. root@laika:~# groupmod -n darts snooker root@laika:~# tail -5 /etc/group tennis:x:1006:inge,katrien football:x:1007: formula1:x:1009:annelies salsa:x:1010:katrien darts:x:1008:sandra 24.6. groupdel You can permanently remove a group with the groupdel command. root@laika:~# groupdel tennis root@laika:~# 24.7. groups A user can type the groups command to see a list of groups where the user belongs to. [harry@RHEL4b ~]$ groups harry sports [harry@RHEL4b ~]$ 209
    • groups 24.8. gpasswd You can delegate control of group membership to another user with the gpasswd command. In the example below we delegate permissions to add and remove group members to serena for the sports group. Then we su to serena and add harry to the sports group. [root@RHEL4b ~]# gpasswd -A serena sports [root@RHEL4b ~]# su - serena [serena@RHEL4b ~]$ id harry uid=516(harry) gid=520(harry) groups=520(harry) [serena@RHEL4b ~]$ gpasswd -a harry sports Adding user harry to group sports [serena@RHEL4b ~]$ id harry uid=516(harry) gid=520(harry) groups=520(harry),522(sports) [serena@RHEL4b ~]$ tail -1 /etc/group sports:x:522:serena,venus,harry [serena@RHEL4b ~]$ Group administrators do not have to be a member of the group. They can remove themselves from a group, but this does not influence their ability to add or remove members. [serena@RHEL4b ~]$ gpasswd -d serena sports Removing user serena from group sports [serena@RHEL4b ~]$ exit Information about group administrators is kept in the /etc/gshadow file. [root@RHEL4b ~]# tail -1 /etc/gshadow sports:!:serena:venus,harry [root@RHEL4b ~]# To remove all group administrators from a group, use the gpasswd command to set an empty administrators list. [root@RHEL4b ~]# gpasswd -A "" sports 24.9. vigr Similar to vipw, the vigr command can be used to manually edit the /etc/group file, since it will do proper locking of the file. Only experienced senior administrators should use vi or vigr to manage groups. 210
    • groups 24.10. practice: groups 1. Create the groups tennis, football and sports. 2. In one command, make venus a member of tennis and sports. 3. Rename the football group to foot. 4. Use vi to add serena to the tennis group. 5. Use the id command to verify that serena is a member of tennis. 6. Make someone responsible for managing group membership of foot and sports. Test that it works. 211
    • groups 24.11. solution: groups 1. Create the groups tennis, football and sports. groupadd tennis ; groupadd football ; groupadd sports 2. In one command, make venus a member of tennis and sports. usermod -a -G tennis,sports venus 3. Rename the football group to foot. groupmod -n foot football 4. Use vi to add serena to the tennis group. vi /etc/group 5. Use the id command to verify that serena is a member of tennis. id (and after logoff logon serena should be member) 6. Make someone responsible for managing group membership of foot and sports. Test that it works. gpasswd -A (to make manager) gpasswd -a (to add member) 212
    • Part VIII. file security
    • Chapter 25. standard file permissions Table of Contents 25.1. 25.2. 25.3. 25.4. 25.5. file ownership ............................................................................................ list of special files ...................................................................................... permissions ................................................................................................. practice: standard file permissions ............................................................. solution: standard file permissions ............................................................. 214 215 216 217 222 223
    • standard file permissions 25.1. file ownership user owner and group owner The users and groups of a system can be locally managed in /etc/passwd and /etc/ group, or they can be in a NIS, LDAP, or Samba domain. These users and groups can own files. Actually, every file has a user owner and a group owner, as can be seen in the following screenshot. paul@RHELv4u4:~/test$ ls -l total 24 -rw-rw-r-- 1 paul paul 17 -rw-rw-r-- 1 paul paul 106 -rw-rw-r-- 1 paul proj 984 -rw-r--r-- 1 root root 0 paul@RHELv4u4:~/test$ Feb Feb Feb Feb 7 5 5 7 11:53 17:04 15:38 16:07 file1 file2 data.odt stuff.txt User paul owns three files, two of those are also owned by the group paul; data.odt is owned by the group proj. The root user owns the file stuff.txt, as does the group root. chgrp You can change the group owner of a file using the chgrp command. root@laika:/home/paul# root@laika:/home/paul# -rw-r--r-- 1 root root root@laika:/home/paul# root@laika:/home/paul# -rw-r--r-- 1 root paul touch FileForPaul ls -l FileForPaul 0 2008-08-06 14:11 FileForPaul chgrp paul FileForPaul ls -l FileForPaul 0 2008-08-06 14:11 FileForPaul chown The user owner of a file can be changed with chown command. root@laika:/home/paul# -rw-r--r-- 1 root paul root@laika:/home/paul# root@laika:/home/paul# -rw-r--r-- 1 paul paul ls -l FileForPaul 0 2008-08-06 14:11 FileForPaul chown paul FileForPaul ls -l FileForPaul 0 2008-08-06 14:11 FileForPaul You can also use chown to change both the user owner and the group owner. root@laika:/home/paul# ls -l FileForPaul -rw-r--r-- 1 paul paul 0 2008-08-06 14:11 FileForPaul root@laika:/home/paul# chown root:project42 FileForPaul root@laika:/home/paul# ls -l FileForPaul -rw-r--r-- 1 root project42 0 2008-08-06 14:11 FileForPaul 215
    • standard file permissions 25.2. list of special files When you use ls -l, for each file you can see ten characters before the user and group owner. The first character tells us the type of file. Regular files get a -, directories get a d, symbolic links are shown with an l, pipes get a p, character devices a c, block devices a b, and sockets an s. Table 25.1. Unix special files first character file type - normal file d directory l symbolic link p named pipe b block device c character device s socket Below a screenshot of a character device (the console) and a block device (the hard disk). paul@debian6lt~$ ls -ld /dev/console /dev/sda crw------1 root root 5, 1 Mar 15 12:45 /dev/console brw-rw---1 root disk 8, 0 Mar 15 12:45 /dev/sda And here you can see a directory, a regular file and a symbolic link. paul@debian6lt~$ ls drwxr-xr-x 128 root -rw-r--r-1 root lrwxrwxrwx 1 root -ld /etc /etc/hosts /etc/motd root 12288 Mar 15 18:34 /etc root 372 Dec 10 17:36 /etc/hosts root 13 Dec 5 10:36 /etc/motd -> /var/run/motd 216
    • standard file permissions 25.3. permissions rwx The nine characters following the file type denote the permissions in three triplets. A permission can be r for read access, w for write access, and x for execute. You need the r permission to list (ls) the contents of a directory. You need the x permission to enter (cd) a directory. You need the w permission to create files in or remove files from a directory. Table 25.2. standard Unix file permissions permission on a file on a directory r (read) read file contents (cat) read directory contents (ls) w (write) change file contents (vi) create files in (touch) x (execute) execute the file enter the directory (cd) three sets of rwx We already know that the output of ls -l starts with ten characters for each file. This screenshot shows a regular file (because the first character is a - ). paul@RHELv4u4:~/test$ ls -l proc42.bash -rwxr-xr-- 1 paul proj 984 Feb 6 12:01 proc42.bash Below is a table describing the function of all ten characters. Table 25.3. Unix file permissions position position characters function 1 - this is a regular file 2-4 rwx permissions for the user owner 5-7 r-x permissions for the group owner 8-10 r-- permissions for others When you are the user owner of a file, then the user owner permissions apply to you. The rest of the permissions have no influence on your access to the file. When you belong to the group that is the group owner of a file, then the group owner permissions apply to you. The rest of the permissions have no influence on your access to the file. When you are not the user owner of a file and you do not belong to the group owner, then the others permissions apply to you. The rest of the permissions have no influence on your access to the file. 217
    • standard file permissions permission examples Some example combinations on files and directories are seen in this screenshot. The name of the file explains the permissions. paul@laika:~/perms$ ls total 12K drwxr-xr-x 2 paul paul -rwxrwxrwx 1 paul paul -r--r----- 1 paul paul -rwxrwx--- 1 paul paul dr-xr-x--- 2 paul paul dr-x------ 2 paul paul paul@laika:~/perms$ -lh 4.0K 0 0 0 4.0K 4.0K 2007-02-07 2007-02-07 2007-02-07 2007-02-07 2007-02-07 2007-02-07 22:26 22:21 22:21 22:21 22:25 22:25 AllEnter_UserCreateDelete EveryoneFullControl.txt OnlyOwnersRead.txt OwnersAll_RestNothing.txt UserAndGroupEnter OnlyUserEnter To summarise, the first rwx triplet represents the permissions for the user owner. The second triplet corresponds to the group owner; it specifies permissions for all members of that group. The third triplet defines permissions for all other users that are not the user owner and are not a member of the group owner. setting permissions (chmod) Permissions can be changed with chmod. The first example gives the user owner execute permissions. paul@laika:~/perms$ ls -l permissions.txt -rw-r--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod u+x permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwxr--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt This example removes the group owners read permission. paul@laika:~/perms$ chmod g-r permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwx---r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt This example removes the others read permission. paul@laika:~/perms$ chmod o-r permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwx------ 1 paul paul 0 2007-02-07 22:34 permissions.txt This example gives all of them the write permission. paul@laika:~/perms$ chmod a+w permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwx-w--w- 1 paul paul 0 2007-02-07 22:34 permissions.txt 218
    • standard file permissions You don't even have to type the a. paul@laika:~/perms$ chmod +x permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwx-wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt You can also set explicit permissions. paul@laika:~/perms$ chmod u=rw permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rw--wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt Feel free to make any kind of combination. paul@laika:~/perms$ chmod u=rw,g=rw,o=r permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt Even fishy combinations are accepted by chmod. paul@laika:~/perms$ chmod u=rwx,ug+rw,o=r permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwxrw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt 219
    • standard file permissions setting octal permissions Most Unix administrators will use the old school octal system to talk about and set permissions. Look at the triplet bitwise, equating r to 4, w to 2, and x to 1. Table 25.4. Octal permissions binary octal permission 000 0 --- 001 1 --x 010 2 -w- 011 3 -wx 100 4 r-- 101 5 r-x 110 6 rw- 111 7 rwx This makes 777 equal to rwxrwxrwx and by the same logic, 654 mean rw-r-xr-- . The chmod command will accept these numbers. paul@laika:~/perms$ chmod 777 permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwxrwxrwx 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod 664 permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt paul@laika:~/perms$ chmod 750 permissions.txt paul@laika:~/perms$ ls -l permissions.txt -rwxr-x--- 1 paul paul 0 2007-02-07 22:34 permissions.txt 220
    • standard file permissions umask When creating a file or directory, a set of default permissions are applied. These default permissions are determined by the umask. The umask specifies permissions that you do not want set on by default. You can display the umask with the umask command. [Harry@RHEL4b 0002 [Harry@RHEL4b [Harry@RHEL4b -rw-rw-r-- 1 [Harry@RHEL4b ~]$ umask ~]$ touch test ~]$ ls -l test Harry Harry 0 Jul 24 06:03 test ~]$ As you can also see, the file is also not executable by default. This is a general security feature among Unixes; newly created files are never executable by default. You have to explicitly do a chmod +x to make a file executable. This also means that the 1 bit in the umask has no meaning--a umask of 0022 is the same as 0033. mkdir -m When creating directories with mkdir you can use the -m option to set the mode. This screenshot explains. paul@debian5~$ mkdir -m 700 MyDir paul@debian5~$ mkdir -m 777 Public paul@debian5~$ ls -dl MyDir/ Public/ drwx------ 2 paul paul 4096 2011-10-16 19:16 MyDir/ drwxrwxrwx 2 paul paul 4096 2011-10-16 19:16 Public/ 221
    • standard file permissions 25.4. practice: standard file permissions 1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there. 2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ? 3. As root, create a file in the users ~/permissions directory. 4. As normal user, look at who owns this file created by root. 5. Change the ownership of all files in ~/permissions to yourself. 6. Make sure you have all rights to these files, and others can only read. 7. With chmod, is 770 the same as rwxrwx--- ? 8. With chmod, is 664 the same as r-xr-xr-- ? 9. With chmod, is 400 the same as r-------- ? 10. With chmod, is 734 the same as rwxr-xr-- ? 11a. Display the umask in octal and in symbolic form. 11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works. 12. Create a file as root, give only read to others. Can a normal user read this file ? Test writing to this file with vi. 13a. Create a file as normal user, give only read to others. Can another normal user read this file ? Test writing to this file with vi. 13b. Can root read this file ? Can root write to this file with vi ? 14. Create a directory that belongs to a group, where every member of that group can read and write to files, and create files. Make sure that people can only delete their own files. 222
    • standard file permissions 25.5. solution: standard file permissions 1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there. mkdir ~/permissions ; touch ~/permissions/myfile.txt 2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ? cp /etc/hosts ~/permissions/ The copy is owned by you. 3. As root, create a file in the users ~/permissions directory. (become root)# touch /home/username/permissions/rootfile 4. As normal user, look at who owns this file created by root. ls -l ~/permissions The file created by root is owned by root. 5. Change the ownership of all files in ~/permissions to yourself. chown user ~/permissions/* You cannot become owner of the file that belongs to root. 6. Make sure you have all rights to these files, and others can only read. chmod 644 (on files) chmod 755 (on directories) 7. With chmod, is 770 the same as rwxrwx--- ? yes 8. With chmod, is 664 the same as r-xr-xr-- ? No 9. With chmod, is 400 the same as r-------- ? yes 10. With chmod, is 734 the same as rwxr-xr-- ? no 11a. Display the umask in octal and in symbolic form. umask ; umask -S 223
    • standard file permissions 11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works. umask -S u=rwx,go= 12. Create a file as root, give only read to others. Can a normal user read this file ? Test writing to this file with vi. (become root) # echo hello > /home/username/root.txt # chmod 744 /home/username/root.txt (become user) vi ~/root.txt 13a. Create a file as normal user, give only read to others. Can another normal user read this file ? Test writing to this file with vi. echo hello > file ; chmod 744 file Yes, others can read this file 13b. Can root read this file ? Can root write to this file with vi ? Yes, root can read and write to this file. Permissions do not apply to root. 14. Create a directory that belongs to a group, where every member of that group can read and write to files, and create files. Make sure that people can only delete their own files. mkdir /home/project42 ; groupadd project42 chgrp project42 /home/project42 ; chmod 775 /home/project42 You can not yet do the last part of this exercise... 224
    • Chapter 26. advanced file permissions Table of Contents 26.1. 26.2. 26.3. 26.4. 26.5. sticky bit on directory ................................................................................ setgid bit on directory ................................................................................ setgid and setuid on regular files ............................................................... practice: sticky, setuid and setgid bits ....................................................... solution: sticky, setuid and setgid bits ....................................................... 225 226 226 227 228 229
    • advanced file permissions 26.1. sticky bit on directory You can set the sticky bit on a directory to prevent users from removing files that they do not own as a user owner. The sticky bit is displayed at the same location as the x permission for others. The sticky bit is represented by a t (meaning x is also there) or a T (when there is no x for others). root@RHELv4u4:~# mkdir /project55 root@RHELv4u4:~# ls -ld /project55 drwxr-xr-x 2 root root 4096 Feb 7 17:38 /project55 root@RHELv4u4:~# chmod +t /project55/ root@RHELv4u4:~# ls -ld /project55 drwxr-xr-t 2 root root 4096 Feb 7 17:38 /project55 root@RHELv4u4:~# The sticky bit can also be set with octal permissions, it is binary 1 in the first of four triplets. root@RHELv4u4:~# chmod 1775 /project55/ root@RHELv4u4:~# ls -ld /project55 drwxrwxr-t 2 root root 4096 Feb 7 17:38 /project55 root@RHELv4u4:~# You will typically find the sticky bit on the /tmp directory. root@barry:~# ls -ld /tmp drwxrwxrwt 6 root root 4096 2009-06-04 19:02 /tmp 26.2. setgid bit on directory setgid can be used on directories to make sure that all files inside the directory are owned by the group owner of the directory. The setgid bit is displayed at the same location as the x permission for group owner. The setgid bit is represented by an s (meaning x is also there) or a S (when there is no x for the group owner). As this example shows, even though root does not belong to the group proj55, the files created by root in /project55 will belong to proj55 since the setgid is set. root@RHELv4u4:~# groupadd proj55 root@RHELv4u4:~# chown root:proj55 /project55/ root@RHELv4u4:~# chmod 2775 /project55/ root@RHELv4u4:~# touch /project55/fromroot.txt root@RHELv4u4:~# ls -ld /project55/ drwxrwsr-x 2 root proj55 4096 Feb 7 17:45 /project55/ root@RHELv4u4:~# ls -l /project55/ total 4 -rw-r--r-- 1 root proj55 0 Feb 7 17:45 fromroot.txt root@RHELv4u4:~# You can use the find command to find all setgid directories. paul@laika:~$ find / -type d -perm -2000 2> /dev/null /var/log/mysql /var/log/news /var/local ... 226
    • advanced file permissions 26.3. setgid and setuid on regular files These two permissions cause an executable file to be executed with the permissions of the file owner instead of the executing owner. This means that if any user executes a program that belongs to the root user, and the setuid bit is set on that program, then the program runs as root. This can be dangerous, but sometimes this is good for security. Take the example of passwords; they are stored in /etc/shadow which is only readable by root. (The root user never needs permissions anyway.) root@RHELv4u4:~# ls -l /etc/shadow -r-------- 1 root root 1260 Jan 21 07:49 /etc/shadow Changing your password requires an update of this file, so how can normal non-root users do this? Let's take a look at the permissions on the /usr/bin/passwd. root@RHELv4u4:~# ls -l /usr/bin/passwd -r-s--x--x 1 root root 21200 Jun 17 2005 /usr/bin/passwd When running the passwd program, you are executing it with root credentials. You can use the find command to find all setuid programs. paul@laika:~$ find /usr/bin -type f -perm -04000 /usr/bin/arping /usr/bin/kgrantpty /usr/bin/newgrp /usr/bin/chfn /usr/bin/sudo /usr/bin/fping6 /usr/bin/passwd /usr/bin/gpasswd ... In most cases, setting the setuid bit on executables is sufficient. Setting the setgid bit will result in these programs to run with the credentials of their group owner. 227
    • advanced file permissions 26.4. practice: sticky, setuid and setgid bits 1a. Set up a directory, owned by the group sports. 1b. Members of the sports group should be able to create files in this directory. 1c. All files created in this directory should be group-owned by the sports group. 1d. Users should be able to delete only their own user-owned files. 1e. Test that this works! 2. Verify the permissions on /usr/bin/passwd. Remove the setuid, then try changing your password as a normal user. Reset the permissions back and try again. 3. If time permits (or if you are waiting for other students to finish this practice), read about file attributes in the man page of chattr and lsattr. Try setting the i attribute on a file and test that it works. 228
    • advanced file permissions 26.5. solution: sticky, setuid and setgid bits 1a. Set up a directory, owned by the group sports. groupadd sports mkdir /home/sports chown root:sports /home/sports 1b. Members of the sports group should be able to create files in this directory. chmod 770 /home/sports 1c. All files created in this directory should be group-owned by the sports group. chmod 2770 /home/sports 1d. Users should be able to delete only their own user-owned files. chmod +t /home/sports 1e. Test that this works! Log in with different users (group members and others and root), create files and watch the permissions. Try changing and deleting files... 2. Verify the permissions on /usr/bin/passwd. Remove the setuid, then try changing your password as a normal user. Reset the permissions back and try again. root@deb503:~# ls -l /usr/bin/passwd -rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd root@deb503:~# chmod 755 /usr/bin/passwd root@deb503:~# ls -l /usr/bin/passwd -rwxr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd A normal user cannot change password now. root@deb503:~# chmod 4755 /usr/bin/passwd root@deb503:~# ls -l /usr/bin/passwd -rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd 3. If time permits (or if you are waiting for other students to finish this practice), read about file attributes in the man page of chattr and lsattr. Try setting the i attribute on a file and test that it works. paul@laika:~$ sudo su [sudo] password for paul: root@laika:~# mkdir attr root@laika:~# cd attr/ root@laika:~/attr# touch file42 root@laika:~/attr# lsattr ------------------ ./file42 root@laika:~/attr# chattr +i file42 229
    • advanced file permissions root@laika:~/attr# lsattr ----i------------- ./file42 root@laika:~/attr# rm -rf file42 rm: cannot remove `file42': Operation not permitted root@laika:~/attr# chattr -i file42 root@laika:~/attr# rm -rf file42 root@laika:~/attr# 230
    • Chapter 27. access control lists Table of Contents 27.1. 27.2. 27.3. 27.4. 27.5. 27.6. 27.7. acl in /etc/fstab ........................................................................................... getfacl ......................................................................................................... setfacl ......................................................................................................... remove an acl entry ................................................................................... remove the complete acl ............................................................................ the acl mask ............................................................................................... eiciel ........................................................................................................... 232 232 232 233 233 233 234 Standard Unix permissions might not be enough for some organisations. This chapter introduces access control lists or acl's to further protect files and directories. 231
    • access control lists 27.1. acl in /etc/fstab File systems that support access control lists, or acls, have to be mounted with the acl option listed in /etc/fstab. In the example below, you can see that the root file system has acl support, whereas /home/data does not. root@laika:~# tail -4 /etc/fstab /dev/sda1 / ext3 /dev/sdb2 /home/data auto pasha:/home/r /home/pasha nfs wolf:/srv/data /home/wolf nfs acl,relatime noacl,defaults defaults defaults 0 0 0 0 1 0 0 0 27.2. getfacl Reading acls can be done with /usr/bin/getfacl. This screenshot shows how to read the acl of file33 with getfacl. paul@laika:~/test$ getfacl file33 # file: file33 # owner: paul # group: paul user::rwgroup::r-mask::rwx other::r-- 27.3. setfacl Writing or changing acls can be done with /usr/bin/setfacl. These screenshots show how to change the acl of file33 with setfacl. First we add user sandra with octal permission 7 to the acl. paul@laika:~/test$ setfacl -m u:sandra:7 file33 Then we add the group tennis with octal permission 6 to the acl of the same file. paul@laika:~/test$ setfacl -m g:tennis:6 file33 The result is visible with getfacl. paul@laika:~/test$ getfacl file33 # file: file33 # owner: paul # group: paul user::rwuser:sandra:rwx group::r-group:tennis:rwmask::rwx other::r-- 232
    • access control lists 27.4. remove an acl entry The -x option of the setfacl command will remove an acl entry from the targeted file. paul@laika:~/test$ paul@laika:~/test$ user:sandra:rwx paul@laika:~/test$ paul@laika:~/test$ setfacl -m u:sandra:7 file33 getfacl file33 | grep sandra setfacl -x sandra file33 getfacl file33 | grep sandra Note that omitting the u or g when defining the acl for an account will default it to a user account. 27.5. remove the complete acl The -b option of the setfacl command will remove the acl from the targeted file. paul@laika:~/test$ setfacl -b file33 paul@laika:~/test$ getfacl file33 # file: file33 # owner: paul # group: paul user::rwgroup::r-other::r-- 27.6. the acl mask The acl mask defines the maximum effective permissions for any entry in the acl. This mask is calculated every time you execute the setfacl or chmod commands. You can prevent the calculation by using the --no-mask switch. paul@laika:~/test$ setfacl --no-mask -m u:sandra:7 file33 paul@laika:~/test$ getfacl file33 # file: file33 # owner: paul # group: paul user::rwuser:sandra:rwx #effective:rwgroup::r-mask::rwother::r-- 233
    • access control lists 27.7. eiciel Desktop users might want to use eiciel to manage acls with a graphical tool. You will need to install eiciel and nautilus-actions to have an extra tab in nautilus to manage acls. paul@laika:~$ sudo aptitude install eiciel nautilus-actions 234
    • Chapter 28. file links Table of Contents 28.1. 28.2. 28.3. 28.4. 28.5. 28.6. 28.7. inodes ......................................................................................................... about directories ......................................................................................... hard links ................................................................................................... symbolic links ............................................................................................ removing links ........................................................................................... practice : links ............................................................................................ solution : links ............................................................................................ 236 237 238 239 239 240 241 An average computer using Linux has a file system with many hard links and symbolic links. To understand links in a file system, you first have to understand what an inode is. 235
    • file links 28.1. inodes inode contents An inode is a data structure that contains metadata about a file. When the file system stores a new file on the hard disk, it stores not only the contents (data) of the file, but also extra properties like the name of the file, the creation date, its permissions, the owner of the file, and more. All this information (except the name of the file and the contents of the file) is stored in the inode of the file. The ls -l command will display some of the inode contents, as seen in this screenshot. root@rhel53 ~# ls -ld /home/project42/ drwxr-xr-x 4 root pro42 4.0K Mar 27 14:29 /home/project42/ inode table The inode table contains all of the inodes and is created when you create the file system (with mkfs). You can use the df -i command to see how many inodes are used and free on mounted file systems. root@rhel53 ~# df -i Filesystem Inodes IUsed IFree IUse% Mounted on /dev/mapper/VolGroup00-LogVol00 4947968 115326 4832642 3% / /dev/hda1 26104 45 26059 1% /boot tmpfs 64417 1 64416 1% /dev/shm /dev/sda1 262144 2207 259937 1% /home/project42 /dev/sdb1 74400 5519 68881 8% /home/project33 /dev/sdb5 0 0 0 - /home/sales /dev/sdb6 100744 11 100733 1% /home/research In the df -i screenshot above you can see the inode usage for several mounted file systems. You don't see numbers for /dev/sdb5 because it is a fat file system. inode number Each inode has a unique number (the inode number). You can see the inode numbers with the ls -li command. paul@RHELv4u4:~/test$ touch file1 paul@RHELv4u4:~/test$ touch file2 paul@RHELv4u4:~/test$ touch file3 paul@RHELv4u4:~/test$ ls -li total 12 817266 -rw-rw-r-- 1 paul paul 0 Feb 817267 -rw-rw-r-- 1 paul paul 0 Feb 817268 -rw-rw-r-- 1 paul paul 0 Feb paul@RHELv4u4:~/test$ 236 5 15:38 file1 5 15:38 file2 5 15:38 file3
    • file links These three files were created one after the other and got three different inodes (the first column). All the information you see with this ls command resides in the inode, except for the filename (which is contained in the directory). inode and file contents Let's put some data in one of the files. paul@RHELv4u4:~/test$ ls -li total 16 817266 -rw-rw-r-- 1 paul paul 0 Feb 817270 -rw-rw-r-- 1 paul paul 92 Feb 817268 -rw-rw-r-- 1 paul paul 0 Feb paul@RHELv4u4:~/test$ cat file2 It is winter now and it is very cold. We do not like the cold, we prefer hot paul@RHELv4u4:~/test$ 5 15:38 file1 5 15:42 file2 5 15:38 file3 summer nights. The data that is displayed by the cat command is not in the inode, but somewhere else on the disk. The inode contains a pointer to that data. 28.2. about directories a directory is a table A directory is a special kind of file that contains a table which maps filenames to inodes. Listing our current directory with ls -ali will display the contents of the directory file. paul@RHELv4u4:~/test$ total 32 817262 drwxrwxr-x 2 800768 drwx------ 16 817266 -rw-rw-r-1 817270 -rw-rw-r-1 817268 -rw-rw-r-1 paul@RHELv4u4:~/test$ ls -ali paul paul paul paul paul paul 4096 Feb paul 4096 Feb paul 0 Feb paul 92 Feb paul 0 Feb 5 5 5 5 5 15:42 15:42 15:38 15:42 15:38 . .. file1 file2 file3 . and .. You can see five names, and the mapping to their five inodes. The dot . is a mapping to itself, and the dotdot .. is a mapping to the parent directory. The three other names are mappings to different inodes. 237
    • file links 28.3. hard links creating hard links When we create a hard link to a file with ln, an extra entry is added in the directory. A new file name is mapped to an existing inode. paul@RHELv4u4:~/test$ ln file2 hardlink_to_file2 paul@RHELv4u4:~/test$ ls -li total 24 817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1 817270 -rw-rw-r-- 2 paul paul 92 Feb 5 15:42 file2 817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3 817270 -rw-rw-r-- 2 paul paul 92 Feb 5 15:42 hardlink_to_file2 paul@RHELv4u4:~/test$ Both files have the same inode, so they will always have the same permissions and the same owner. Both files will have the same content. Actually, both files are equal now, meaning you can safely remove the original file, the hardlinked file will remain. The inode contains a counter, counting the number of hard links to itself. When the counter drops to zero, then the inode is emptied. finding hard links You can use the find command to look for files with a certain inode. The screenshot below shows how to search for all filenames that point to inode 817270. Remember that an inode number is unique to its partition. paul@RHELv4u4:~/test$ find / -inum 817270 2> /dev/null /home/paul/test/file2 /home/paul/test/hardlink_to_file2 238
    • file links 28.4. symbolic links Symbolic links (sometimes called soft links) do not link to inodes, but create a name to name mapping. Symbolic links are created with ln -s. As you can see below, the symbolic link gets an inode of its own. paul@RHELv4u4:~/test$ ln -s file2 symlink_to_file2 paul@RHELv4u4:~/test$ ls -li total 32 817273 -rw-rw-r-- 1 paul paul 13 Feb 5 17:06 file1 817270 -rw-rw-r-- 2 paul paul 106 Feb 5 17:04 file2 817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3 817270 -rw-rw-r-- 2 paul paul 106 Feb 5 17:04 hardlink_to_file2 817267 lrwxrwxrwx 1 paul paul 5 Feb 5 16:55 symlink_to_file2 -> file2 paul@RHELv4u4:~/test$ Permissions on a symbolic link have no meaning, since the permissions of the target apply. Hard links are limited to their own partition (because they point to an inode), symbolic links can link anywhere (other file systems, even networked). 28.5. removing links Links can be removed with rm. paul@laika:~$ paul@laika:~$ paul@laika:~$ paul@laika:~$ paul@laika:~$ touch data.txt ln -s data.txt sl_data.txt ln data.txt hl_data.txt rm sl_data.txt rm hl_data.txt 239
    • file links 28.6. practice : links 1. Create two files named winter.txt and summer.txt, put some text in them. 2. Create a hard link to winter.txt named hlwinter.txt. 3. Display the inode numbers of these three files, the hard links should have the same inode. 4. Use the find command to list the two hardlinked files 5. Everything about a file is in the inode, except two things : name them! 6. Create a symbolic link to summer.txt called slsummer.txt. 7. Find all files with inode number 2. What does this information tell you ? 8. Look at the directories /etc/init.d/ /etc/rc.d/ /etc/rc3.d/ ... do you see the links ? 9. Look in /lib with ls -l... 10. Use find to look in your home directory for regular files that do not(!) have one hard link. 240
    • file links 28.7. solution : links 1. Create two files named winter.txt and summer.txt, put some text in them. echo cold > winter.txt ; echo hot > summer.txt 2. Create a hard link to winter.txt named hlwinter.txt. ln winter.txt hlwinter.txt 3. Display the inode numbers of these three files, the hard links should have the same inode. ls -li winter.txt summer.txt hlwinter.txt 4. Use the find command to list the two hardlinked files find . -inum xyz 5. Everything about a file is in the inode, except two things : name them! The name of the file is in a directory, and the contents is somewhere on the disk. 6. Create a symbolic link to summer.txt called slsummer.txt. ln -s summer.txt slsummer.txt 7. Find all files with inode number 2. What does this information tell you ? It tells you there is more than one inode table (one for every formatted partition + virtual file systems) 8. Look at the directories /etc/init.d/ /etc/rc.d/ /etc/rc3.d/ ... do you see the links ? ls -l /etc/init.d ls -l /etc/rc.d ls -l /etc/rc3.d 9. Look in /lib with ls -l... ls -l /lib 10. Use find to look in your home directory for regular files that do not(!) have one hard link. find ~ ! -links 1 -type f 241
    • Part IX. Appendices
    • Appendix A. certifications A.1. Certification LPI: Linux Professional Institute LPIC Level 1 This is the junior level certification. You need to pass exams 101 and 102 to achieve LPIC 1 certification. To pass level one, you will need Linux command line, user management, backup and restore, installation, networking, and basic system administration skills. LPIC Level 2 This is the advanced level certification. You need to be LPIC 1 certified and pass exams 201 and 202 to achieve LPIC 2 certification. To pass level two, you will need to be able to administer medium sized Linux networks, including Samba, mail, news, proxy, firewall, web, and ftp servers. LPIC Level 3 This is the senior level certification. It contains one core exam (301) which tests advanced skills mainly about ldap. To achieve this level you also need LPIC Level 2 and pass a specialty exam (302 or 303). Exam 302 mainly focuses on Samba, and 303 on advanced security. More info on http://www.lpi.org. Ubuntu When you are LPIC Level 1 certified, you can take a LPI Ubuntu exam (199) and become Ubuntu certified. Red Hat Certified Engineer The big difference with most other certifications is that there are no multiple choice questions for RHCE. Red Hat Certified Engineers have to take a live exam consisting of two parts. First, they have to troubleshoot and maintain an existing but broken setup (scoring at least 80 percent), and second they have to install and configure a machine (scoring at least 70 percent). 243
    • certifications MySQL There are two tracks for MySQL certification; Certified MySQL 5.0 Developer (CMDEV) and Certified MySQL 5.0 DBA (CMDBA). The CMDEV is focused towards database application developers, and the CMDBA towards database administrators. Both tracks require two exams each. The MySQL cluster DBA certification requires CMDBA certification and passing the CMCDBA exam. Novell CLP/CLE To become a Novell Certified Linux Professional, you have to take a live practicum. This is a VNC session to a set of real SLES servers. You have to perform several tasks and are free to choose your method (commandline or YaST or ...). No multiple choice involved. Sun Solaris Sun uses the classical formula of multiple choice exams for certification. Passing two exams for an operating system gets you the Solaris Certified Administrator for Solaris X title. Other certifications There are many other lesser known certifications like EC council's Certified Ethical Hacker, CompTIA's Linux+, and Sair's Linux GNU. 244
    • Appendix B. keyboard settings B.1. about keyboard layout Many people (like US-Americans) prefer the default US-qwerty keyboard layout. So when you are not from the USA and want a local keyboard layout on your system, then the best practice is to select this keyboard at installation time. Then the keyboard layout will always be correct. Also, whenever you use ssh to remotely manage a linux system, your local keyboard layout will be used, independent of the server keyboard configuration. So you will not find much information on changing keyboard layout on the fly on linux, because not many people need it. Below are some tips to help you. B.2. X Keyboard Layout This is the relevant portion in /etc/X11/xorg.conf, first for Belgian azerty, then for US-qwerty. [paul@RHEL5 ~]$ grep -i xkb /etc/X11/xorg.conf Option "XkbModel" "pc105" Option "XkbLayout" "be" [paul@RHEL5 ~]$ grep -i xkb /etc/X11/xorg.conf Option "XkbModel" "pc105" Option "XkbLayout" "us" When in Gnome or KDE or any other graphical environment, look in the graphical menu in preferences, there will be a keyboard section to choose your layout. Use the graphical menu instead of editing xorg.conf. B.3. shell keyboard layout When in bash, take a look in the /etc/sysconfig/keyboard file. Below a sample USqwerty configuration, followed by a Belgian azerty configuration. [paul@RHEL5 ~]$ cat /etc/sysconfig/keyboard KEYBOARDTYPE="pc" KEYTABLE="us" [paul@RHEL5 ~]$ cat /etc/sysconfig/keyboard KEYBOARDTYPE="pc" KEYTABLE="be-latin1" The keymaps themselves can be found in /usr/share/keymaps or /lib/kbd/keymaps. 245
    • keyboard settings [paul@RHEL5 ~]$ ls -l /lib/kbd/keymaps/ total 52 drwxr-xr-x 2 root root 4096 Apr 1 00:14 drwxr-xr-x 2 root root 4096 Apr 1 00:14 drwxr-xr-x 8 root root 4096 Apr 1 00:14 drwxr-xr-x 2 root root 4096 Apr 1 00:14 drwxr-xr-x 4 root root 4096 Apr 1 00:14 lrwxrwxrwx 1 root root 3 Apr 1 00:14 drwxr-xr-x 2 root root 4096 Apr 1 00:14 246 amiga atari i386 include mac ppc -> mac sun
    • Appendix C. hardware C.1. buses about buses Hardware components communicate with the Central Processing Unit or cpu over a bus. The most common buses today are usb, pci, agp, pci-express and pcmcia aka pc-card. These are all Plag and Play buses. Older x86 computers often had isa buses, which can be configured using jumpers or dip switches. /proc/bus To list the buses recognised by the Linux kernel on your computer, look at the contents of the /proc/bus/ directory (screenshot from Ubuntu 7.04 and RHEL4u4 below). root@laika:~# ls /proc/bus/ input pccard pci usb [root@RHEL4b ~]# ls /proc/bus/ input pci usb Can you guess which of these two screenshots was taken on a laptop ? /usr/sbin/lsusb To list all the usb devices connected to your system, you could read the contents of /proc/bus/usb/devices (if it exists) or you could use the more readable output of lsusb, which is executed here on a SPARC system with Ubuntu. root@shaka:~# lsusb Bus 001 Device 002: ID 0430:0100 Sun Microsystems, Inc. 3-button Mouse Bus 001 Device 003: ID 0430:0005 Sun Microsystems, Inc. Type 6 Keyboard Bus 001 Device 001: ID 04b0:0136 Nikon Corp. Coolpix 7900 (storage) root@shaka:~# /var/lib/usbutils/usb.ids The /var/lib/usbutils/usb.ids file contains a gzipped list of all known usb devices. 247
    • hardware paul@barry:~$ zmore /var/lib/usbutils/usb.ids | head ------> /var/lib/usbutils/usb.ids <-----# # List of USB ID's # # Maintained by Vojtech Pavlik <vojtech@suse.cz> # If you have any new entries, send them to the maintainer. # The latest version can be obtained from # http://www.linux-usb.org/usb.ids # # $Id: usb.ids,v 1.225 2006/07/13 04:18:02 dbrownell Exp $ /usr/sbin/lspci To get a list of all pci devices connected, you could take a look at /proc/bus/pci or run lspci (partial output below). paul@laika:~$ lspci ... 00:06.0 FireWire (IEEE 1394): Texas Instruments TSB43AB22/A IEEE-139... 00:08.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-816... 00:09.0 Multimedia controller: Philips Semiconductors SAA7133/SAA713... 00:0a.0 Network controller: RaLink RT2500 802.11g Cardbus/mini-PCI 00:0f.0 RAID bus controller: VIA Technologies, Inc. VIA VT6420 SATA ... 00:0f.1 IDE interface: VIA Technologies, Inc. VT82C586A/B/VT82C686/A... 00:10.0 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1.... 00:10.1 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1.... ... C.2. interrupts about interrupts An interrupt request or IRQ is a request from a device to the CPU. A device raises an interrupt when it requires the attention of the CPU (could be because the device has data ready to be read by the CPU). Since the introduction of pci, irq's can be shared among devices. Interrupt 0 is always reserved for the timer, interrupt 1 for the keyboard. IRQ 2 is used as a channel for IRQ's 8 to 15, and thus is the same as IRQ 9. /proc/interrupts You can see a listing of interrupts on your system in /proc/interrupts. paul@laika:~$ cat /proc/interrupts 248
    • hardware CPU0 0: 1320048 1: 10224 7: 0 8: 2 10: 3062 12: 131 15: 47073 18: 0 19: 31056 20: 19042 21: 44052 22: 188352 23: 632444 24: 1585 CPU1 555 7 0 1 21 2 0 1 1 1 1 1 1 1 IO-APIC-edge IO-APIC-edge IO-APIC-edge IO-APIC-edge IO-APIC-fasteoi IO-APIC-edge IO-APIC-edge IO-APIC-fasteoi IO-APIC-fasteoi IO-APIC-fasteoi IO-APIC-fasteoi IO-APIC-fasteoi IO-APIC-fasteoi IO-APIC-fasteoi timer i8042 parport0 rtc acpi i8042 ide1 yenta libata, ohci1394 eth0 uhci_hcd:usb1, uhci_hcd:usb2,... ra0 nvidia VIA82XX-MODEM, VIA8237 dmesg You can also use dmesg to find irq's allocated at boot time. paul@laika:~$ dmesg | grep "irq 1[45]" [ 28.930069] ata3: PATA max UDMA/133 cmd 0x1f0 ctl 0x3f6 bmdma 0x2090 irq 14 [ 28.930071] ata4: PATA max UDMA/133 cmd 0x170 ctl 0x376 bmdma 0x2098 irq 15 C.3. io ports about io ports Communication in the other direction, from CPU to device, happens through IO ports. The CPU writes data or control codes to the IO port of the device. But this is not only a one way communication, the CPU can also use a device's IO port to read status information about the device. Unlike interrupts, ports cannot be shared! /proc/ioports You can see a listing of your system's IO ports via /proc/ioports. [root@RHEL4b ~]# cat /proc/ioports 0000-001f : dma1 0020-0021 : pic1 0040-0043 : timer0 0050-0053 : timer1 0060-006f : keyboard 0070-0077 : rtc 0080-008f : dma page reg 00a0-00a1 : pic2 00c0-00df : dma2 00f0-00ff : fpu 0170-0177 : ide1 02f8-02ff : serial ... 249
    • hardware C.4. dma about dma A device that needs a lot of data, interrupts and ports can pose a heavy load on the cpu. With dma or Direct Memory Access a device can gain (temporary) access to a specific range of the ram memory. /proc/dma Looking at /proc/dma might not give you the information that you want, since it only contains currently assigned dma channels for isa devices. root@laika:~# cat /proc/dma 1: parport0 4: cascade pci devices that are using dma are not listed in /proc/dma, in this case dmesg can be useful. The screenshot below shows that during boot the parallel port received dma channel 1, and the Infrared port received dma channel 3. root@laika:~# dmesg | egrep -C 1 'dma 1|dma 3' [ 20.576000] parport: PnPBIOS parport detected. [ 20.580000] parport0: PC-style at 0x378 (0x778), irq 7, dma 1... [ 20.764000] irda_init() -[ 21.204000] pnp: Device 00:0b activated. [ 21.204000] nsc_ircc_pnp_probe() : From PnP, found firbase 0x2F8... [ 21.204000] nsc-ircc, chip->init 250
    • Appendix D. License GNU Free Documentation License Version 1.3, 3 November 2008 Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. 0. PREAMBLE The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others. This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software. We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference. 1. APPLICABILITY AND DEFINITIONS This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The "Document", below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as "you". You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law. A "Modified Version" of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language. A "Secondary Section" is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them. The "Invariant Sections" are certain Secondary Sections whose titles 251
    • License are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none. The "Cover Texts" are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words. A "Transparent" copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not "Transparent" is called "Opaque". Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only. The "Title Page" means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, "Title Page" means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text. The "publisher" means any person or entity that distributes copies of the Document to the public. A section "Entitled XYZ" means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as "Acknowledgements", "Dedications", "Endorsements", or "History".) To "Preserve the Title" of such a section when you modify the Document means that it remains a section "Entitled XYZ" according to this definition. The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License. 2. VERBATIM COPYING You may copy and distribute the Document in any medium, either 252
    • License commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3. You may also lend copies, under the same conditions stated above, and you may publicly display copies. 3. COPYING IN QUANTITY If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects. If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages. If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public. It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document. 4. MODIFICATIONS You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version: * A. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission. 253
    • License * B. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement. * C. State on the Title page the name of the publisher of the Modified Version, as the publisher. * D. Preserve all the copyright notices of the Document. * E. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices. * F. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below. * G. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice. * H. Include an unaltered copy of this License. * I. Preserve the section Entitled "History", Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled "History" in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence. * J. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the "History" section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission. * K. For any section Entitled "Acknowledgements" or "Dedications", Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein. * L. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles. * M. Delete any section Entitled "Endorsements". Such a section may not be included in the Modified Version. * N. Do not retitle any existing section to be Entitled "Endorsements" or to conflict in title with any Invariant Section. * O. Preserve any Warranty Disclaimers. If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles. You may add a section Entitled "Endorsements", provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard. You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, 254
    • License you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one. The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version. 5. COMBINING DOCUMENTS You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers. The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work. In the combination, you must combine any sections Entitled "History" in the various original documents, forming one section Entitled "History"; likewise combine any sections Entitled "Acknowledgements", and any sections Entitled "Dedications". You must delete all sections Entitled "Endorsements". 6. COLLECTIONS OF DOCUMENTS You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects. You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document. 7. AGGREGATION WITH INDEPENDENT WORKS A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an "aggregate" if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document. If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate. 255
    • License 8. TRANSLATION Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail. If a section in the Document is Entitled "Acknowledgements", "Dedications", or "History", the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title. 9. TERMINATION You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License. However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation. Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice. Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it. 10. FUTURE REVISIONS OF THIS LICENSE The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/. Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies 256
    • License that a proxy can decide which future versions of this License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Document. 11. RELICENSING "Massive Multiauthor Collaboration Site" (or "MMC Site") means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A "Massive Multiauthor Collaboration" (or "MMC") contained in the site means any set of copyrightable works thus published on the MMC site. "CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization. "Incorporate" means to publish or republish a Document, in whole or in part, as part of another Document. An MMC is "eligible for relicensing" if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008. The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing. 257
    • Index Symbols ; (shell), 84 !! (shell), 101 ! (bash history), 101 ! (file globbing), 108 ? (file globbing), 107 /, 28, 52 /bin, 53, 76 /bin/bash, 73, 204 /bin/cat, 53 /bin/csh, 73 /bin/date, 53 /bin/ksh, 73, 204 /bin/rm, 77 /bin/sh, 73 /boot, 55 /boot/grub, 55 /boot/grub/grub.cfg, 55 /boot/grub/grub.conf, 55 /dev, 36, 59 /dev/null, 59, 117 /dev/pts/1, 59 /dev/random, 70 /dev/tty1, 59 /dev/urandom, 69, 71 /dev/zero, 70 /etc, 55 /etc/bashrc, 205 /etc/default/useradd, 190 /etc/fstab, 232 /etc/group, 208, 215 /etc/gshadow, 210 /etc/hosts, 71 /etc/init.d/, 55 /etc/inputrc, 204 /etc/login.defs, 194 /etc/passwd, 132, 189, 195, 195, 197, 215 /etc/profile, 204 /etc/resolv.conf, 71 /etc/shadow, 191, 193, 227 /etc/shells, 158, 197 /etc/skel, 56, 196 /etc/sudoers, 199, 199 /etc/sysconfig, 56 /etc/sysconfig/firstboot, 56 /etc/sysconfig/harddisks, 56 /etc/sysconfig/hwconf, 56 /etc/sysconfig/keyboard, 56 /etc/X11/xorg.conf, 55 /export, 57 /home, 57 /lib, 54 /lib/kbd/keymaps/, 56 /lib/modules, 54 /lib32, 54 /lib64, 54 /media, 57 /opt, 54 /proc, 36, 60 /proc/bus, 247 /proc/bus/pci, 248 /proc/bus/usb/devices, 247 /proc/cpuinfo, 61 /proc/dma, 250 /proc/interrupts, 62, 248 /proc/ioports, 249 /proc/kcore, 62 /proc/sys, 61 /root, 57 /run, 67 /sbin, 53, 76 /srv, 57 /sys, 63 /tmp, 58, 226 /usr, 64 /usr/bin, 64 /usr/bin/getfacl, 232 /usr/bin/passwd, 227 /usr/bin/setfacl, 232 /usr/include, 64 /usr/lib, 64 /usr/local, 64 /usr/share, 65 /usr/share/games, 65 /usr/share/man, 65 /usr/src, 65 /var, 66 /var/cache, 66 /var/lib, 67 /var/lib/rpm, 67 /var/lib/usbutils/usb.ids, 247 /var/lock, 67 /var/log, 66 258
    • Index /var/log/messages, 66 /var/log/syslog, 66 /var/run, 67 /var/spool, 67 /var/tmp, 67 ., 27 .., 27 .. (directory), 237 . (directory), 237 . (shell), 159 .bash_history, 102 .bash_login, 204 .bash_logout, 206 .bash_profile, 204 .bashrc, 204, 205 .exrc, 153 .vimrc, 153 `(backtick), 96 ~, 27 '(single quote), 96 " (double quotes), 75 (( (shell), 179 -- (shell), 160 [ (file globbing), 107 [ (shell), 164 $? (shell variables), 84 $() embedded shell, 96 $ (shell variables), 90 $HISTFILE, 102 $HISTFILESIZE, 102 $HISTSIZE, 102 $LANG, 108 $PATH, 76, 91 $PS1, 28 * (file globbing), 107 (backslash), 86 &, 84 &&, 85 #!/bin/bash, 158 #! (shell), 158 # (pound sign), 86 >, 115 >>, 116 >|, 116 |, 120 ||, 85 1>, 117 2>, 117 2>&1, 117 777, 220 A access control list, 232 acl, 234 acls, 232 agp, 247 AIX, 3 alias(bash), 77 alias(shell), 77 apropos, 23 arguments(shell), 74 B backticks, 96 base64, 118 bash, 171 bash history, 101 bash -x, 160 binaries, 53 Bourne again shell, 73 BSD, 3 bunzip2, 141 bus, 247 bzcat, 141 bzip2, 140, 141, 141 bzmore, 141 C cal, 139 case, 181 case sensitive, 36 cat, 124 cat(1), 46 cd(bash builtin), 27 cd -(bash builtin), 28 CentOS, 5 chage(1), 194 chgrp(1), 215 chkconfig, 56 chmod, 196, 220 chmod(1), 150, 218 chmod +x, 158, 221 chown, 196 chown(1), 215 chsh(1), 197 CMDBA, 244 CMDEV, 244 259
    • Index comm(1), 129 command line scan, 74 command mode(vi), 147 copyleft, 8 copyright, 7, 7 cp(1), 38, 38 cpu, 247 crypt, 192 csh, 158 Ctrl d, 46 ctrl-r, 102 current directory, 27 cut, 132 cut(1), 126 D daemon, 23 date, 138 Debian, 5 Dennis Ritchie, 3 devfs, 63 df -i, 236 directory, 237 distribution, 4 distributions, 52 dma, 250 dmesg(1), 249, 250 dumpkeys(1), 56 E echo, 74 echo(1), 74, 75 echo $-, 95 echo *, 109 Edubuntu, 5 eiciel, 234 ELF, 54 elif, 165 embedding(shell), 96 env(1), 93, 93 environment variable, 90 EOF, 118 escaping (shell), 109 eval, 179 executables, 53 exit (bash), 102 export, 93 F Fedora, 5 FHS, 52 file(1), 36, 54 file globbing, 106 file ownership, 215 Filesystem Hierarchy Standard, 52 filters, 123 find(1), 137, 226, 227, 238 FireWire, 63 for (bash), 165 FOSS, 7 four freedoms, 8 Free Software, 7 free software, 8 freeware, 7 function (shell), 182 G gcc(1), 193 getfacl, 232 getopts, 174 GID, 208 glob(7), 107 GNU, 3 gpasswd, 210 GPL, 8 GPLv3, 8 grep(1), 124 grep -i, 124 grep -v, 125 groupadd(1), 208 groupdel(1), 209 groupmod(1), 209 groups, 208 groups(1), 209 gunzip(1), 140 gzip, 140 gzip(1), 140 H hard link, 238 head(1), 45 here directive, 47 here document, 118 here string, 118 hidden files, 29 HP, 3 260
    • Index HP-UX, 3 http://www.pathname.com/fhs/, 52 I IBM, 3 id(1), 188 IEEE 1394, 63 if then else (bash), 165 inode, 235, 238 inode table, 236 insert mode(vi), 147 interrupt, 248 IO Ports, 249 IRQ, 248 isa, 247 K Ken Thompson, 3 kernel, 54 keymaps(5), 56 Korn shell, 103 Korn Shell, 197 ksh, 103, 158 kudzu, 56 L man hier, 52 man -k, 23 md5, 193 mkdir, 196 mkdir(1), 31, 221 mkdir -p, 31 mkfs, 236 more(1), 48 mv(1), 39 N noclobber, 115 nounset(shell), 94 Novell Certified Linux Professional, 244 O octal permissions, 220 od(1), 130 OEL, 5 open source, 8 open source definition, 8 open source software, 7 openssl(1), 192 Oracle Enterprise Linux, 5 owner, 217 less(1), 48 let, 180 Linus Torvalds, 3 Linux Mint, 5 ln, 239 ln(1), 238 loadkeys(1), 56 locate(1), 138 logical AND, 85 logical OR, 85 Logiciel Libre, 8 LPIC 1 Certification, 243 LPIC 2 Certification, 243 ls, 217, 236 ls(1), 29, 29, 236, 237 ls -l, 216 lspci, 248 lsusb, 247 P M random number generator, 70 read, 172 reboot, 102 Red Hat, 5 magic(5), 36 man(1), 23, 24, 24 mandb(1), 25 parent directory, 27 passwd, 194 passwd(1), 24, 191, 191, 192, 227 passwd(5), 24 path, 28, 29 pc-card, 247 pci, 247 pci-express, 247 pcmcia, 247 pipe, 120 popd, 34 primary group, 190 proprietary, 7 public domain, 7 pushd, 34 pwd(1), 27, 28 R 261
    • Index regular expressions, 103 rename(1), 40 repository, 4 RHCE, 243 Richard Stallman, 3 rm(1), 37, 239 rmdir(1), 31 rmdir -p, 31 rm -rf, 38 root, 53, 189, 198, 199, 199 root directory, 52 rpm, 67 S salt (encryption), 193 Scientific, 5 sed, 131 set, 95 set(shell), 92 set +x, 78 setfacl, 232 setgid, 226, 226 setuid, 160, 199, 227, 227 set -x, 78 she-bang (shell), 158 shell, 204 shell comment, 86 shell escaping, 86 shell expansion, 74, 74 shell functions, 182 shift, 172 shopt, 175 skeleton, 56 sleep, 139 soft link, 239 Solaris, 3 sort, 132 sort(1), 128 source, 159, 173 stderr, 115 stdin, 115, 120, 124 stdout, 115, 120, 124 sticky bit, 226 strings(1), 48 su, 195, 210 su -, 91 su(1), 198, 198 sudo, 195, 199 sudo(1), 199 sudo su -, 200 Sun, 3 SunOS, 3 superuser, 189 symbolic link, 239 sysfs, 63 System V, 54 T tab key(bash), 29 tac(1), 47 tail(1), 45 tee(1), 124 test, 164 time, 139 touch(1), 37 tr, 127 tr(1), 126 type(shell), 76 U Ubuntu, 5 umask(1), 221 unalias(bash), 78 uniq, 132 uniq(1), 129 Unix, 3 unset, 95 unset(shell), 92 until (bash), 166 updatedb(1), 138 usb, 63, 247 useradd, 190, 196 useradd(1), 192, 196 useradd -D, 190 userdel(1), 190 usermod, 209 usermod(1), 190, 194, 195 V vi, 210 vi(1), 146 vigr(1), 210 vim(1), 146 vimtutor(1), 146 vipw(1), 195 visudo(1), 199 vrije software, 8 262
    • Index W w(1), 188 wc(1), 127 whatis(1), 23 whereis(1), 24 which(1), 76 while (bash), 166 white space(shell), 74 who, 132 who(1), 188 who am i, 188 whoami(1), 188 wild cards, 108 X X, 55 X Window System, 55 Z zcat, 140 zmore, 140 263